Prepare upload to Galera 26.4.20 to Debian 11 "Bullseye"

See commits for details. Imported with same commands as in !22.

There are no Debian tracked bugs at https://tracker.debian.org/pkg/galera-4 that needs fixing in this oldstable update, thus this upstream minor release is contains purely upstream bugfixes. Uploads to Debian unstable and stable-proposed has already been made and no regressions encountered so far.

Git history shows merge from upstream git tag onto branch upstream/latest, and tarball import into it:

Changes are easy to review locally with git difftool --dir-diff debian/26.4.18-0+deb11u1:

A stable update bug report was sent in https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1080364 but seems there are no Bullseye updates anymore, need to follow the process at https://lts-team.pages.debian.net/ instead.