feat!: remove "is_merge_allowed" job

The "is_merge_allowed" job has no value/benefits any more.
It was used to ensure, that merges into the default branch were only allowed from next.
This was required to trigger the other jobs since they were checking for "next" as the source branch.

By now the job rules check for any merges into a protected branch. Therefore the name of the source branch is irrelevant.

BREAKING CHANGE: The is_merge_allowed job is obsolete and thus removed.