Release 17.6

[rainersigwald]

• When VS main snaps to 17.7, create vs17.6 branch
• Create darc channel for VS 17.7 if it doesn't already exist
  darc add-channel --name "VS 17.7"
• Ping internal "First Responders" Teams channel to get the new channel made available as a promotion target (e.g. Make "VS 17.6" darc channel promotionable arcade#12150): Make VS 17.7 and 17.8 promotable arcade#12989
• Remove the main to old release channel default channel
  darc delete-default-channel --repo https://github.com/dotnet/msbuild --branch main --channel "VS 17.6"
• Associate the main branch with the next release channel
  darc add-default-channel --channel "VS 17.7" --branch main --repo https://github.com/dotnet/msbuild
• Check subscriptions for the current channel VS 17.6 and update as necessary (for instance, SDK's main branch should usually be updated, whereas release branches often should not be
  darc get-subscriptions --exact --source-repo https://github.com/dotnet/msbuild --channel "VS 17.6"
  Update channel VS 17.6 to VS 17.7 for the sdk main subscription
  darc update-subscription --id 5480c5ca-4fa6-4d8a-1987-08d9d606f30d
• Ensure that the current release channel VS 17.6 is associated with the correct release branch
  darc get-default-channels --source-repo https://github.com/dotnet/msbuild --branch vs17.6
  if it is not, darc add-default-channel --channel "VS 17.6" --branch vs17.6 --repo https://github.com/dotnet/msbuild
• Fast-forward merge the correct commit (the one that is currently inserted to VS main) to the vs17.6 branch
  git push upstream 797fd829a374a880f63fd4eea4ea6990404c48f4:refs/heads/vs17.6
• Update the branch merge flow in dotnet/versions to have the currently-in-servicing branches (extend Automate MSBuild's merge chain versions#867): MSBuild 17.6->16.7 merges versions#875
• Fix OptProf data flow for the new vs17.6 branch
• Update the release-branch insertion release definition to have InsertTargetBranch rel/d17.6.
• Prepare final branding PR for vs17.6 but ⚠️ DO NOT MERGE IT YET ⚠️: 17.6 final branding #8604
• Create 17.7 branding PR, including public API baseline package version change (see Public api analyzer change #8116 (comment)): Bump version to 17.7 #8612
• Merge 17.7 branding PR
• Prepare QB-mode bug for final branding insertion in internal VS repo
• Update internal "create VS PR for insertion" release to point to the new VS release branch by default
• Merge final branding to vs17.6 branch
• Get QB approval
• Merge to VS
• ASAP after release, push packages to nuget.org (not currently automated, contact dnceng)
• Update main subscriptions to the new channel (this can be done after release)
  darc get-subscriptions --exact --target-repo https://github.com/dotnet/msbuild --target-branch main
• Create the 17.6 release
• Update the PackageValidationBaselineVersion to the latest released version (17.6.0)