Update milestone assignments for PRs

[dougbu]

• rules would have assigned PRs to released milestones
  • note however that these rules don't presently seem to be working!!

nits:

• remove duplicate rule for release/6.0 PRs
• remove rule for release/3.1 PRs since the release is no EOL

[dougbu]

@wtgodbe I think I saw you in a conversation that relates to the source-build error below. Has the underlying issue been resolved❔ Mostly wondering when to azp run again.

  Entering 'src/submodules/MessagePack-CSharp'
  Cloning repository at: . -> /__w/1/s/artifacts/source-build/self/src/src/submodules/MessagePack-CSharp ...
  Destination already exists!
  Clean is enabled: removing /__w/1/s/artifacts/source-build/self/src/src/submodules/MessagePack-CSharp and continuing...
  Creating empty clone at: /__w/1/s/artifacts/source-build/self/src/src/submodules/MessagePack-CSharp
  Cloning into '/__w/1/s/artifacts/source-build/self/src/src/submodules/MessagePack-CSharp'...
  fatal: transport 'file' not allowed
  fatal: run_command returned non-zero status for src/submodules/MessagePack-CSharp
  .
/__w/1/s/.packages/microsoft.dotnet.arcade.sdk/8.0.0-beta.23063.7/tools/SourceBuild/SourceBuildArcadeBuild.targets(170,5): error MSB3073: The command "git submodule foreach --recursive '/__w/1/s/.packages/microsoft.dotnet.arcade.sdk/8.0.0-beta.23063.7/tools/SourceBuild/git-clone-to-dir.sh  --source . --dest "/__w/1/s/artifacts/source-build/self/src/$sm_path" --clean'" exited with code 128. [/__w/1/s/.packages/microsoft.dotnet.arcade.sdk/8.0.0-beta.23063.7/tools/Build.proj]

[wtgodbe]

It's an issue with the build agents, not the code in this repo: dotnet/source-build#3111. It's about whether or not your build gets assigned a new enough agent - so just hit "rerun" until it works, for now.