ci(vs-build): adapt to Visual Studio 2026 default on windows-latest

The `windows-latest` runner image migration that began on June 8, 2026
and completes on June 15, 2026 switches the default Visual Studio
install from VS 2022 (v17) to VS 2026 (v18), per
actions/runner-images#14017.

CMake 4.x picks up the new generator name "Visual Studio 18 2026"
automatically and, crucially, writes the solution file with the new
`.slnx` (XML) extension rather than `.sln`. See
https://github.com/Kitware/CMake/blob/v4.3.2/Source/cmGlobalVisualStudioGenerator.cxx#L1147-L1159
where `GetSLNFile()` appends an "x" to the filename when the generator
version is `VS18` or newer.

As a result, the `MSBuild` step in the `vs-build` job fails with

	MSBUILD : error MSB1009: Project file does not exist.
	Switch: git.sln

because the file CMake actually wrote is `git.slnx`. An example of the
failure can be seen at
https://github.com/git-for-windows/git/actions/runs/27264770241/job/80556419519.
Update the step to reference `git.slnx` accordingly; MSBuild 18 (which
the new image ships) understands the XML solution format natively.

Additionally, drop the `-property:PlatformToolset=v142` argument that
had been carried since 889cacb (ci: configure GitHub Actions for
CI/PR, 2020-04-11), when this job was first configured for VS 2019.
The VS 2026 install on `windows-latest` only ships the v144 toolset
along with a v143 compatibility component
(`Microsoft.VisualStudio.Component.VC.14.44.17.14.x86.x64`); v142 is
no longer present, so the explicit pin would now also fail in its own
right. Removing it lets MSBuild use whatever toolset CMake selected
during configuration (v144 in the current image), which keeps the
configure and build steps consistent with each other.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Assisted-by: Opus 4.7

Author: dscho

.github/workflows/main.yml

@@ -199,7 +199,7 @@ jobs:
         cmake `pwd`/contrib/buildsystems/ -DCMAKE_PREFIX_PATH=`pwd`/compat/vcbuild/vcpkg/installed/${{ matrix.arch }}-windows \
         -DNO_GETTEXT=YesPlease -DPERL_TESTS=OFF -DPYTHON_TESTS=OFF -DCURL_NO_CURL_CMAKE=ON -DCMAKE_GENERATOR_PLATFORM=${{ matrix.arch }} -DVCPKG_ARCH=${{ matrix.arch }}-windows -DHOST_CPU=${{ matrix.arch }}
     - name: MSBuild
-      run: msbuild git.sln -property:Configuration=Release -property:Platform=${{ matrix.arch }} -maxCpuCount:4 -property:PlatformToolset=v142
+      run: msbuild git.slnx -property:Configuration=Release -property:Platform=${{ matrix.arch }} -maxCpuCount:4
     - name: bundle artifact tar
       shell: bash
       env: