Stop syncing main-ub branch

[ellahathaway]

Fixes dotnet/source-build#4090

As part of this change, pipeline triggers in AzDO dnceng-public/public and dnceng/internal need to be updated. The default branch needs to be updated as well.

[ellahathaway]

As part of this change, pipeline triggers in AzDO dnceng-public/public and dnceng/internal need to be updated. The default branch needs to be updated as well.

Marking as a draft while I work to address this.

Edit: This is a list of links to the triggers and default branches that need to be updated once this PR is merged:

• dotnet-unified-build internal default branch (internal Microsoft link)
• dotnet-unified-build internal pipeline triggers (internal Microsoft link)
• dotnet-unified build public default branch (internal Microsoft link)
• dotnet-unified-build public pipeline triggers (internal Microsoft link)
  • Need to change default branch for scheduled trigger as well (internal Microsoft link)

[ViktorHofer]

Updated all the triggers, the branch policies on GH for dotnet/dotnet and the branch policy on AzDO for the dotnet-dotnet repo.

[ellahathaway]

Updated all the triggers, the branch policies on GH for dotnet/dotnet and the branch policy on AzDO for the dotnet-dotnet repo.

Thanks, Viktor!