release-windows.yml: changed solution extension to slnx (fixes build)

this is caused by `vs2025` now using Visual Studio 2026

Author: firewave

.github/workflows/release-windows.yml

@@ -50,7 +50,7 @@ jobs:
           cd pcre-%PCRE_VERSION% || exit /b !errorlevel!
           git apply --ignore-space-change ..\externals\pcre.patch || exit /b !errorlevel!
           cmake . -A x64 -DPCRE_BUILD_PCRECPP=OFF -DPCRE_BUILD_PCREGREP=OFF -DPCRE_BUILD_TESTS=OFF -DCMAKE_POLICY_VERSION_MINIMUM=3.5 -DCMAKE_COMPILE_WARNING_AS_ERROR=On || exit /b !errorlevel!
-          msbuild -m PCRE.sln -p:Configuration=Release -p:Platform=x64 || exit /b !errorlevel!
+          msbuild -m PCRE.slnx -p:Configuration=Release -p:Platform=x64 || exit /b !errorlevel!
           copy pcre.h ..\externals || exit /b !errorlevel!
           copy Release\pcre.lib ..\externals\pcre64.lib || exit /b !errorlevel!
 
@@ -99,7 +99,7 @@ jobs:
 
       # TODO: build with multiple threads
       - name: Build CLI x64 release configuration using MSBuild
-        run: msbuild -m cppcheck.sln -t:cli -p:Configuration=Release-PCRE -p:Platform=x64 -p:HaveBoost=HAVE_BOOST -p:BoostInclude=%GITHUB_WORKSPACE%\boost || exit /b !errorlevel!
+        run: msbuild -m cppcheck.slnx -t:cli -p:Configuration=Release-PCRE -p:Platform=x64 -p:HaveBoost=HAVE_BOOST -p:BoostInclude=%GITHUB_WORKSPACE%\boost || exit /b !errorlevel!
         env:
           _CL_: /WX