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.

Since pre-release testing is usually carried out for branches in which CHANGES entries are intentionally malformed to prevent entry numbering conflicts down the road, do not run the "changes" GitLab CI job in pipelines that are triggered by a parent pipeline (which can currently only be a pre-release testing pipeline) to prevent triggering job failures that would be meaningless anyway.