Allow bumping versions after release branch cut

[kitallis]

Closes: https://github.com/orgs/tramlinehq/projects/5/views/1?pane=issue&itemId=112406191

Why

We currently support bumping up versions before the release branch is cut. This forces the new release branch to have the correct current version before it kicks off and allows for flows where the versions aren't managed by Tramline through workflows alone.

This addresses

This PR adds a mandatory selection of bumping strategies, including a new one, that allows updating the version in the working branch after the release branch has been cut to the next version that is yet to be released. This is also a common flow for some users, since their subsequent feature PRs rely on the next version instead of the currently releasing one.

It does not block the release from progressing –

And has slightly different text and description in the PR, example PR.

Scenarios tested

• All previous trains with version bump enabled, default to the current release version after branch cut
• Previous strategy continues to work
• New strategy does not block the release branch work
• New strategy PR cut can be closed/merged as usual
• New strategy version bump PR is asynchronously created