Remove stale branches

[Artur-Romaniuk]

Branch 1-alpha is stale and has been merged in some way to main, so it can be deleted now.
On the similar note, when pulling repo locally I noticed that there still exists master branch which was used in the past. The existence of both master and main can be confusing, as one has to check which one of them is actively used.

[dbrgn]

We should remove notes about the branch, but we should not delete it, since some projects might still have that branch in their dependencies. And we don't want to break those builds unnecessarily.