Do not trigger GitLab Pages pipelines any more

The bind.isc.org microsite no longer downloads documentation from
GitLab, instead linking to Read the Docs for all currently maintained
branches.  This makes it redundant to trigger GitLab Pages pipelines
after each merge to any of the maintained branches.  Revert changes
introduced by commit 31bde118db: remove
the "push:docs" job along with the "push" stage it is associated with
and revert artifact expiry time for the "docs" job to the default value.

(cherry picked from commit a7fccaee82)

.gitlab-ci.yml

@@ -65,7 +65,6 @@ stages:
   - system
   - performance
   - docs
-  - push
   - postcheck
   - release
 
@@ -527,7 +526,6 @@ tarball-create:
       artifacts: true
 
 # Jobs for doc builds on Debian 11 "bullseye" (amd64)
-# Long "docs" job artifact retention is needed for bind.isc.org web site
 
 docs:
   <<: *release_branch_triggering_rules
@@ -540,7 +538,6 @@ docs:
       artifacts: true
   artifacts:
     untracked: true
-    expire_in: "1 month"
 
 docs:tarball:
   <<: *default_triggering_rules
@@ -553,18 +550,6 @@ docs:tarball:
     - job: tarball-create
       artifacts: true
 
-push:docs:
-  <<: *base_image
-  stage: push
-  needs:
-    - job: docs
-      artifacts: false
-  script:
-    - curl -X POST -F token=$GITLAB_PAGES_DOCS_TRIGGER_TOKEN -F ref=main $GITLAB_PAGES_DOCS_TRIGGER_URL
-  only:
-    - main@isc-projects/bind9
-    - /^v9_[1-9][0-9]$/@isc-projects/bind9
-
 # Jobs for regular GCC builds on Alpine Linux 3.15 (amd64)
 
 gcc:alpine3.15:amd64: