Added C++23 testing to MSYS2 jobs.

Author: Lastique

.github/workflows/ci.yml

@@ -644,8 +644,8 @@ jobs:
       fail-fast: false
       matrix:
         include:
-          - { sys: MINGW32, toolset: gcc, cxxstd: '11,17,20' }
-          - { sys: MINGW64, toolset: gcc, cxxstd: '11,17,20' }
+          - { sys: MINGW32, toolset: gcc, cxxstd: '11,17,20,23' }
+          - { sys: MINGW64, toolset: gcc, cxxstd: '11,17,20,23' }
 
     runs-on: windows-latest