Retire Ubuntu 18.04 CI builder

The GitHub-hosted Ubuntu 18.04 has been deprecated and will be
entirely unsupported as of April 2023. Leading up to this there
will be scheduled "brownouts" to encourage users to update their
workflows.

This commit retires our use of the GitHub-hosted Ubuntu 18.04
runners in advance of their removal.

Reviewed-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Damian Szuberski <szuberskidamian@gmail.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #14238