Merge bitcoin/bitcoin#23060: release: increase minimum compiler and lib(std)c++ requirements

182de7b ci: update minimum compiler requirements for std::filesystem (fanquake)
04f5baf doc: update minimum compiler requirements for std::filesystem (fanquake)

Pull request description:

  This increases the minimum required compiler versions to Clang 7 and GCC 8.1. This has been split out of #20744 (migration to `std::filesystem`), as it's also a requirement for some other changes, such as #20452 or #20457 which want to make use of `std::from_chars`. As well as #20435, which is also `std::filesystem` related. Given that the `std::filesystem` changes are moving ahead, splitting out this change to let other PRs take advantage of the new requirements seems worthwhile.

  Clang 7 has been available in Debian since [Stretch (oldoldstable)](https://packages.debian.org/stretch/clang-7) and in Ubuntu since [Bionic (18.04)](https://packages.ubuntu.com/bionic-updates/clang-7). GCC 8 has been available in Debian since [Buster (oldstable)](https://packages.debian.org/buster/gcc) and in Ubuntu since [Bionic (18.04)](https://packages.ubuntu.com/bionic/gcc-8). CentOS 8 also packages GCC 8.

  The CI changes here give us one build with GCC 8, and another using Clang 7 on top of libc++.

  Note that the minimum required libc++ in dependencies.md is unchanged as, at least for `<filesystem>`, and the `*_chars` use cases, libc++ 7 [should be sufficient](https://en.cppreference.com/w/cpp/compiler_support/17).

  I've tested that building `<filesystem>` code using Clang 7 & libc++ works. i.e `clang++-7 -std=c++17 fs.cpp -stdlib=libc++ -lc++fs`. Also that building `<filesystem>` code with Clang 7 and libstdc++ 8 works. i.e `clang++-7 -std=c++17  fs.cpp -lstdc++fs`.

ACKs for top commit:
  MarcoFalke:
    review ACK 182de7b

Tree-SHA512: 5bc151c4be58005711eed6bd8a091f3417f75a0218c11c08cffff9d749edadd965726bb7856a8e693e96e69ed0596989cda1aac4b29fb6d30705b1687a5b3363

Author: fanquake

ci/test/00_setup_env_native_nowallet.sh

@@ -8,7 +8,7 @@ export LC_ALL=C.UTF-8
 
 export CONTAINER_NAME=ci_native_nowallet
 export DOCKER_NAME_TAG=ubuntu:18.04  # Use bionic to have one config run the tests in python3.6, see doc/dependencies.md
-export PACKAGES="python3-zmq clang-5.0 llvm-5.0"  # Use clang-5 to test C++17 compatibility, see doc/dependencies.md
-export DEP_OPTS="NO_WALLET=1"
+export PACKAGES="python3-zmq clang-7 llvm-7 libc++abi-7-dev libc++-7-dev"  # Use clang-7 to test C++17 compatibility, see doc/dependencies.md
+export DEP_OPTS="NO_WALLET=1 CC=clang-7 CXX='clang++-7 -stdlib=libc++'"
 export GOAL="install"
-export BITCOIN_CONFIG="--enable-reduce-exports CC=clang-5.0 CXX=clang++-5.0"
+export BITCOIN_CONFIG="--enable-reduce-exports CC=clang-7 CXX='clang++-7 -stdlib=libc++'"

ci/test/00_setup_env_native_qt5.sh

@@ -7,13 +7,13 @@
 export LC_ALL=C.UTF-8
 
 export CONTAINER_NAME=ci_native_qt5
-export DOCKER_NAME_TAG=ubuntu:18.04  # Check that bionic gcc-7 can compile our c++17 and run our functional tests in python3, see doc/dependencies.md
-export PACKAGES="python3-zmq qtbase5-dev qttools5-dev-tools libdbus-1-dev libharfbuzz-dev"
+export DOCKER_NAME_TAG=ubuntu:18.04  # Check that bionic gcc-8 can compile our C++17 and run our functional tests in python3, see doc/dependencies.md
+export PACKAGES="gcc-8 g++-8 python3-zmq qtbase5-dev qttools5-dev-tools libdbus-1-dev libharfbuzz-dev"
 export DEP_OPTS="NO_QT=1 NO_UPNP=1 NO_NATPMP=1 DEBUG=1 ALLOW_HOST_PACKAGES=1"
 export TEST_RUNNER_EXTRA="--previous-releases --coverage --extended --exclude feature_dbcrash"  # Run extended tests so that coverage does not fail, but exclude the very slow dbcrash
 export RUN_UNIT_TESTS_SEQUENTIAL="true"
 export RUN_UNIT_TESTS="false"
 export GOAL="install"
 export PREVIOUS_RELEASES_TO_DOWNLOAD="v0.15.2 v0.16.3 v0.17.2 v0.18.1 v0.19.1 v0.20.1"
 export BITCOIN_CONFIG="--enable-zmq --with-libs=no --with-gui=qt5 --enable-reduce-exports
---enable-debug --disable-fuzz-binary  CFLAGS=\"-g0 -O2 -funsigned-char\" CXXFLAGS=\"-g0 -O2 -funsigned-char\""
+--enable-debug --disable-fuzz-binary  CFLAGS=\"-g0 -O2 -funsigned-char\" CXXFLAGS=\"-g0 -O2 -funsigned-char\" CC=gcc-8 CXX=g++-8"

doc/dependencies.md

@@ -7,11 +7,11 @@ These are the dependencies currently used by Bitcoin Core. You can find instruct
 | --- | --- | --- | --- | --- | --- |
 | Berkeley DB | [4.8.30](https://www.oracle.com/technetwork/database/database-technologies/berkeleydb/downloads/index.html) | 4.8.x | No |  |  |
 | Boost | [1.71.0](https://www.boost.org/users/download/) | [1.64.0](https://github.com/bitcoin/bitcoin/pull/22320) | No |  |  |
-| Clang<sup>[ \* ](#note1)</sup> |  | [5.0+](https://releases.llvm.org/download.html) (C++17 support) |  |  |  |
+| Clang<sup>[ \* ](#note1)</sup> |  | [7.0](https://releases.llvm.org/download.html) (C++17 & std::filesystem support) |  |  |  |
 | Expat | [2.2.7](https://libexpat.github.io/) |  | No | Yes |  |
 | fontconfig | [2.12.1](https://www.freedesktop.org/software/fontconfig/release/) |  | No | Yes |  |
 | FreeType | [2.7.1](https://download.savannah.gnu.org/releases/freetype) |  | No |  | [Yes](https://github.com/bitcoin/bitcoin/blob/master/depends/packages/qt.mk) (Android only) |
-| GCC |  | [7+](https://gcc.gnu.org/) (C++17 support) |  |  |  |
+| GCC |  | [8.1](https://gcc.gnu.org/) (C++17 & std::filesystem support) |  |  |  |
 | glibc | | [2.17](https://www.gnu.org/software/libc/) |  |  |  |  |
 | HarfBuzz-NG |  |  |  |  | [Yes](https://github.com/bitcoin/bitcoin/blob/master/depends/packages/qt.mk) |
 | libevent | [2.1.12-stable](https://github.com/libevent/libevent/releases) | [2.0.21](https://github.com/bitcoin/bitcoin/pull/18676) | No |  |  |