Drop post-merge pipelines

Commit 24961f6068 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.

(cherry picked from commit 7b8c9e04fc)

.gitlab-ci.yml

@@ -198,17 +198,6 @@ stages:
     - web
     - schedules
 
-.release-branch-triggering-rules: &release_branch_triggering_rules
-  only:
-    - api
-    - merge_requests
-    - tags
-    - triggers
-    - web
-    - schedules
-    - main@isc-projects/bind9
-    - /^v9_[1-9][0-9]$/@isc-projects/bind9
-
 .schedules-tags-web-triggering-rules: &schedules_tags_web_triggering_rules
   only:
     - api
@@ -223,7 +212,7 @@ stages:
   stage: precheck
 
 .autoconf: &autoconf_job
-  <<: *release_branch_triggering_rules
+  <<: *default_triggering_rules
   <<: *base_image
   stage: autoconf
   script:
@@ -528,7 +517,7 @@ tarball-create:
 # Jobs for doc builds on Debian 11 "bullseye" (amd64)
 
 docs:
-  <<: *release_branch_triggering_rules
+  <<: *default_triggering_rules
   <<: *base_image
   <<: *docs_job
   needs: