Finishing PHP 7.4 transition

[Bryce Harrington]

The last three packages blocking php-defaults from migrating were
cleared today:

1.  php-sabredav is removed from the archive, along with
    kopano-webapp-files that was its only rdepend.  sabredav has not
    been maintained upstream for some years, and has several issues due
    to functionality deprecated in php7.4 and a py2/py3 issue.
    (Thanks Doko)

2.  php-league-commonmark had a failing test case on s390x.  We're
    now skipping that test on s390x to let this package succeed.
    (Thanks again Christian!)

3.  phpunit-comparator had a simple typo in one of its tests, which
    php7.4 helpfully warned about.  I upstreamed the fix and landed it
    as a patch in focal.  Debian hits the same issue, so I let them know
    of the fix as well.

The update excuses page still lists the prior versions for #2 and #3,
but I've just retriggered both, and in theory they should be gone next
time the page updates.

Last task will be removing php 7.3, which I'll do once I've confirmed
php-defaults has migrated.

Bryce