Create template sync PRs from versioned branch

[grst]

Fix #396

Manual intervention required before making a release:

• for all template repos that merged the v0.5.0 update PR, manually reset the template-update-v2 branch to the last commit created by scverse-bot, effectively removing all commits that were merged in from the main branch.

[github-actions]

A PR has been generated to the instance repo:
scverse/cookiecutter-scverse-instance#203

You can check out the PR to preview your changes
in an instance of the cookiecutter template. The PR will be kept in sync with
this PR automatically.

[grst]

As far as I can tell this now does what I think it should.
Example PRs created with the version from this PR:

• Update template to test8 Boehringer-Ingelheim/dso#122
• Update template to test8 scirpy#615

Note that now the PR is not crated from the template-sync-v2 branch, but from a template-sync-v2-<tag-name> branch.

[flying-sheep]

for all template repos that merged the v0.5.0 update PR, manually reset the template-update-v2 branch to the last commit created by scverse-bot, effectively removing all commits that were merged in from the main branch.

I don’t quite get it, the branches live in here, right? can’t we just nuke all branches from the bot?

[grst]

yes, this needs to be done in the bot account.

I think nuking the branches entirely would have the disadvantage that git doesn't know that this branch has already been merged which would lead to merge conflict showing up that have already been resolved (I think).

[flying-sheep]

Looks great! I went over it and didn’t find anything that looks wrong.