Allow configuring the update branch prefix

[vcali]

Scala Steward currently uses update as the fixed prefix for update branch names, for example update/cats-effect-3.4.0.

Some installations need to use a different branch namespace for bot-created branches, such as scala-steward/..., to match branch protection, automation, or repository conventions.

I opened a PR that adds a repository-specific configuration option:

pullRequests.branchPrefix = "scala-steward"

PR: #3871

The PR preserves the existing default prefix of update and includes tests and docs.

[mzuehlke]

The wish to make changes to the branch has been raise multiple times over the years.

• Branch named "update" breaks pull requests #741
• Feature to limit branch name length #3329
• Feature Idea: Allow customization of the branch names in the repo configuration #2127

The first time it was turned down and the reason is still valid.

Steward use the branch name to search for already existing PRs. But if we document this problem, that users are aware that PR for the same change will be opened again. I am not against solving this issue.

[vcali]

Thanks @mzuehlke
I'd like if we could document this in order to enable this feature.

Anything I can do to help with that?