Update Microsoft.Build package references

[Redth]

Summary

• Update Microsoft.Build package references from 17.14.28 to 18.6.3
• Add the matching System.Reflection.Metadata package reference used by the build task project

Validation

• dotnet restore src/Microsoft.Android.Build.BaseTasks/Microsoft.Android.Build.BaseTasks.csproj --disable-parallel --verbosity minimal
• dotnet build src/Microsoft.Android.Build.BaseTasks/Microsoft.Android.Build.BaseTasks.csproj --no-restore --verbosity minimal
• dotnet restore tests/Microsoft.Android.Build.BaseTasks-Tests/Microsoft.Android.Build.BaseTasks-Tests.csproj --disable-parallel --verbosity minimal
• dotnet test tests/Microsoft.Android.Build.BaseTasks-Tests/Microsoft.Android.Build.BaseTasks-Tests.csproj --no-restore --verbosity minimal

[Copilot]

Pull request overview

This PR updates the centralized MSBuild package references used by the BaseTasks project and its tests, aligning them with the newer Microsoft.Build dependency set.

Changes:

• Bumps Microsoft.Build-related package versions from 17.14.28 to 18.6.3.
• Adds an explicit centralized System.Reflection.Metadata package version and reference.