mobile: fix iOS build for open source contributors (#39649)

**Commit Message:** mobile: fix iOS build for open source contributors

**Additional Description:**
Me and another contributor were unable to build iOS from our machines as
external contributors. This fixes the build.

~**1) XCode version:** The main change was to update the XCode version
to ~16.3~ 16.1, since 15.3 is too old and no longer runs on modern
macOS.~ Stopped forcing the old XCode version 15.3, allowing the
developer to build on modern machines.

~**2) CI macos version:** The `macos-14` machine doesn't have XCode
16.3. We could use 16.2, but developers should not be using macOS 14 for
security reasons, so migrating to `macos-15` better aligns with the
developer environment.~. _Keeping macos-14 for now to make it work with
the CI_

**3) Broken macros:** We also had to remove the C++17 override for iOS,
since it was causing variadic macros to fail. Adding
`--cxxopt=-Wno-error=variadic-macro-arguments-omitted` fixes the C++17
issue, but using C++20 is more sane and avoids loads of warnings.

**4) MINIMUM_IOS_VERSION:** We also update `MINIMUM_IOS_VERSION` from
16.3 to 16.5. Without that the build was failing with:

```
In file included from external/envoy/source/common/json/json_streamer.h:11:

external/envoy/source/common/buffer/buffer_util.h:46:40: error: 'to_chars' is unavailable: introduced in iOS 16.5 simulator

   46 |     std::to_chars_result result = std::to_chars(buf, buf + sizeof(buf), number);
```

So the support for 16.3 was already absent.

**5) Simulator version:** Stopped forcing the simulator version, which
prevents running on platforms that don't have the exact version of the
simulator required.


**Risk Level:** Medium

**Testing:**

Built the xcframework:

```sh
./bazelw build ios_dist --config=ios
```

Ran the Swift app:

```
./bazelw run //examples/swift/hello_world:app --config=ios
```

<img width="367" alt="image"
src="https://github.com/user-attachments/assets/1114e625-e140-4673-acab-6bd332888463"
/>



Ran the Objective-C app:
```
./bazelw run //examples/swift/hello_world:app --config=ios
```

I'm counting on the CI to run the tests.

**Docs Changes:** None

**Release Notes:** None

**Platform Specific Features:** No new features.

[Optional Runtime guard:]
[Optional Fixes #Issue]
[Optional Fixes commit #PR or SHA]
[Optional Deprecated:]
[Optional [API
Considerations](https://github.com/envoyproxy/envoy/blob/main/api/review_checklist.md):]

---------

Signed-off-by: Vinicius Fortuna <[email protected]>

Author: fortuna

mobile/.bazelrc

@@ -21,8 +21,8 @@ build --features=swift.cacheable_swiftmodules
 build --features=swift.debug_prefix_map
 build --host_force_python=PY3
 build --macos_minimum_os=13.4
-build --ios_minimum_os=16.3
-build --ios_simulator_version=17.4
+# Use MINIMUM_IOS_VERSION in config.bzl instead
+# build --ios_minimum_os=16.3
 build --verbose_failures
 build --workspace_status_command=../bazel/get_workspace_status
 build --use_top_level_targets_for_symlinks

mobile/bazel/config.bzl

@@ -1,3 +1,3 @@
 """Shared configuration for things we don't want to duplicate"""
 
-MINIMUM_IOS_VERSION = "16.3"
+MINIMUM_IOS_VERSION = "16.5"

mobile/ci/BUILD

@@ -3,9 +3,9 @@ licenses(["notice"])  # Apache 2
 xcode_version(
     name = "xcode_16_1_0",
     default_ios_sdk_version = "18.1",
-    default_macos_sdk_version = "14.4",
-    default_tvos_sdk_version = "17.4",
-    default_watchos_sdk_version = "10.4",
+    default_macos_sdk_version = "15.1",
+    default_tvos_sdk_version = "18.1",
+    default_watchos_sdk_version = "11.1",
     version = "16.1",
 )