Include net7.0 TFM for MSBuildLocator

[mthalman]

The dependency flow of source-build-externals to roslyn is failing due to #204. See error info at dotnet/roslyn#69870 (comment). This is because roslyn builds with both net8.0 and net7.0 in source-build CI.

This is fixed by updating the target frameworks to include net7.0.

[mthalman]

Getting a prebuilt because it's trying to use the latest 7.0 targeting pack:

.packages/microsoft.dotnet.arcade.sdk/8.0.0-beta.23451.1/tools/SourceBuild/AfterSourceBuild.proj(68,5): error : (NETCORE_ENGINEERING_TELEMETRY=AfterSourceBuild) 1 new pre-builts discovered! Detailed usage report can be found at /__w/1/s/artifacts/source-build/self/prebuilt-report/baseline-comparison.xml.
See https://aka.ms/dotnet/prebuilts for guidance on what pre-builts are and how to eliminate them.
Package IDs are:
Microsoft.NETCore.App.Ref.7.0.9