If 6.3.0-M1..RC1 exist but 6.3.0 does not, scheduling the next release milestone fails.

> Task :scheduleNextRelease FAILED

FAILURE: Build failed with an exception.

* What went wrong:
Execution failed for task ':scheduleNextRelease'.
> 422[{"message":"Validation Failed","errors":[{"resource":"Milestone","code":"already_exists","field":"title"}],"documentation_url":"https://docs.github.com/rest/issues/milestones#create-a-milestone"}]

Instead, it should ignore already created milestones and create the next one if it does not exist.

Context

The existing behavior is based on the assumption that creating one or more milestones in a release train means the entire train is being created manually, which is not always the case.