Extract CHANGES checks to a separate GitLab CI job

Checking the contents of the CHANGES file currently requires invoking
multiple shell scripts.  These invocations are conflated with those for
other test scripts in the "misc" GitLab CI job.  Extract the commands
checking the contents of the CHANGES file to a separate GitLab CI job,
"changes", to improve readability.  Remove similar checks for the
CHANGES.SE file altogether as they are only relevant for BIND -S and
therefore should not be present in an open source branch.

.gitlab-ci.yml

@@ -504,15 +504,6 @@ misc:
   <<: *precheck_job
   script:
     - sh util/checklibs.sh > checklibs.out
-    - sh util/tabify-changes < CHANGES > CHANGES.tmp
-    - diff -urNap CHANGES CHANGES.tmp
-    - perl util/check-changes CHANGES
-    - sh util/check-line-length.sh CHANGES
-    - test ! -f CHANGES.SE || sh util/tabify-changes < CHANGES.SE > CHANGES.tmp
-    - test ! -f CHANGES.SE || diff -urNap CHANGES.SE CHANGES.tmp
-    - test ! -f CHANGES.SE || perl util/check-changes master=0 CHANGES.SE
-    - test ! -f CHANGES.SE || sh util/check-line-length.sh CHANGES.SE
-    - rm CHANGES.tmp
     - sh util/check-categories.sh
     - sh util/check-gitignore.sh
     - sh util/check-trailing-whitespace.sh
@@ -525,6 +516,16 @@ misc:
       - checklibs.out
     when: on_failure
 
+changes:
+  <<: *precheck_job
+  script:
+    - sh util/tabify-changes < CHANGES > CHANGES.tmp
+    - diff -urNap CHANGES CHANGES.tmp
+    - perl util/check-changes CHANGES
+    - sh util/check-line-length.sh CHANGES
+    - rm CHANGES.tmp
+  needs: []
+
 black:
   <<: *precheck_job
   needs: []