The CODEOWNERS was added almost 3 years ago but never saw any adoption.
Only one person used it (me) to mark what files I maintain and for which
I wish to review commits. No other maintainers or code paths were added,
so clean it away for clarity.
Many recent PRs offer up shell scripts or example queries in the testing
section, when these could be easily and usefully be converted to automated
tests based on MTR. (https://github.com/MariaDB/server/pull/1888 and
https://github.com/MariaDB/server/pull/1940 are good examples of this.)
This modifies the PR template to more strongly encourage the contribution of
automated tests.
