Create dev17.9.x branch

[tmeschter]

Descriptions of these steps can be found in the team OneNote.

• [x] Identify base commit for new branch
  • [x] According to the schedule, main snapped to rel/d17.9 at 12pm PT on 1/5.
  • [x] Last completed Project System insertion PR: https://dev.azure.com/devdiv/DevDiv/_git/VS/pullrequest/519370
  • [x] Last inserted GitHub PR: https://github.com/dotnet/project-system/pull/9358
  • [x] Merge commit of that PR: e617a12f5f3e1e77ddcd7bffcbe292c706a8affe
• [x] Create branch on GitHub at that commit
  • [x] https://github.com/dotnet/project-system/tree/dev17.9.x
• [ ] For branches not matching dev* or feature/* (usually we skip these)
  • [ ] Update the pull-request.yml (via pr) to support PR builds
  • [ ] Update the official.yml (via trigger) to have signed builds the new branch
• [ ] Update Roslyn Tools config.xml file to flow branch changes to the latest dev branch
  • [ ] dotnet/roslyn-tools PR: https://github.com/dotnet/roslyn-tools/pull/❓
• [ ] Update version.json (via "version") to match the version of VS, if needed
  • [x] ~~In new branch: https://github.com/dotnet/project-system/blob/dev17.9.x/version.json~~ Nothing to do here.
  • [ ] In main: https://github.com/dotnet/project-system/blob/main/version.json: #9367
• [ ] To run manual insertions, use the DotNet-Project-System pipeline. Enter the GitHub branch as Branch/tag, the VS branch as VS Insertion Branch Name, and check Create VS Insertion PR.
• [ ] Update MSFTBot milestone tracking