[ci] Build on Fedora 43 and Ubuntu 25.10 with C++23

Following the precedent set for macOS 26 and Debian 13, use the newest
available C++ standard for newly released platforms.

Author: guitargeek

.github/workflows/root-ci-config/buildconfig/fedora43.txt

@@ -1,3 +1,4 @@
+CMAKE_CXX_STANDARD=23
 builtin_zlib=ON
 builtin_zstd=ON
 experimental_adaptivecpp=ON

.github/workflows/root-ci-config/buildconfig/ubuntu2510.txt

@@ -0,0 +1 @@
+CMAKE_CXX_STANDARD=23

.github/workflows/root-ci.yml

@@ -379,7 +379,6 @@ jobs:
           - image: fedora42
             overrides: ["CMAKE_CXX_STANDARD=23"]
           - image: fedora43
-            overrides: ["CMAKE_CXX_STANDARD=23"]
           - image: alma8
           - image: alma9
             overrides: ["CMAKE_BUILD_TYPE=Debug"]
@@ -391,7 +390,6 @@ jobs:
           - image: ubuntu2504
             overrides: ["CMAKE_CXX_STANDARD=23"]
           - image: ubuntu2510
-            overrides: ["CMAKE_CXX_STANDARD=23"]
           - image: debian125
             overrides: ["CMAKE_CXX_STANDARD=20", "dev=ON", "CMAKE_CXX_FLAGS=-Wsuggest-override"]
           - image: debian13