Use highest base for charms

Updates to check if the charm being promoted uses 22.04 as its base, and promote that
one instead of 20.04

Change-Id: Ia5e34419729b1a2969eb4c10981aeeecc76fe544
Signed-off-by: Mark Beierl <mark.beierl@canonical.com>
(cherry picked from commit 2dc01891)