Drop post-merge pipelines
Commit 1c8c1815 introduced the concept of post-merge pipelines. These were used to build documentation after each merge to any of the maintained branches, to allow up-to-date documentation to be subsequently published on the bind.isc.org microsite. As the latter no longer downloads documentation from GitLab, instead linking to Read the Docs for all currently maintained branches, post-merge pipelines have become redundant since running the "autoreconf" and "docs" jobs after each merge is not necessary any more. Drop the "release_branch_triggering_rules" YAML anchor and revert to default triggering rules for the "autoconf" and "docs" jobs, effectively preventing pipelines from being created after each merge to any of the maintained branches.