CI: Retire Ubuntu 20.04 builder

Ubuntu 20.04 has gone EOL as of April 2025, retire the CI builder.

Reviewed-by: Tino Reichardt <milky-zfs@mcmilk.de>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #17403