Do not ask to fix the target branch if it's the next development branch (not yet created)

[chalasr]

During the stabilization phase of a minor version including feature-freeze and beta/RC releases, submitted features are expected to target the next minor version i.e. if we are stabilizing 7.1, we want feature PRs to target 7.2.
In such periods, when a PR really targets the wanna-be-stable branch but indicates to target the next one, carsonbot should not consider it a mismatch - and so should not ask to change the target branch, since the contributor just cannot make its PR target the right branch at the moment (they have to come back and do it later, when the branch is created).

Example: symfony/symfony#56831

[stof]

The reason why we don't create the next development branch at the beginning of the feature freeze but only when publishing the RC release is to focus contributions on stabilizing the upcoming release instead of working on the next one.

So I would not change things here.