ci: bump microsoft/setup-msbuild from v2 to v3

ci: bump microsoft/setup-msbuild from v2 to v3

The v2 of `microsoft/setup-msbuild` runs on Node.js 20, which GitHub
is phasing out of the Actions runners. v3 is a minimal release whose
only substantive change is moving the action's runtime to Node.js 24,
so that our Visual Studio build jobs keep working once Node.js 20 is
removed from the runners.

The risk of this bump is very low: v3 contains no functional changes
to the action itself -- it merely adds `msbuild.exe` to `PATH`, with
no change to command-line flags, inputs, outputs, or default tool
resolution. The only precondition is a recent-enough Actions Runner,
which the github.com-hosted runners already satisfy.

See also:

- Release notes: https://github.com/microsoft/setup-msbuild/releases
- Compare: https://github.com/microsoft/setup-msbuild/compare/v2...v3

Originally-authored-by: dependabot[bot] <support@github.com>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>