pull-updates: Land Bullseye updates to the default branch

No need to tweak the destination branch every time we branch a new release, just target the default branch.

This is not 100% the same thing: packages that got dropped at some point will have the default branch set to a different release (for instance, apertis/v2021).

In those cases, either:

1. no debian/bullseye branch exist, so this is not an issue
2. if we want to land the debian/bullseye updates somewhere, trying with a non-existent branch name is not very useful anyway