Put back mysql-* 8.0.19 track for focal

It's probably too ambitious to try to remove the 8.0.19 track as all the
stable charms on focal refer to it (despite 8.0.22 being the version in
focal now).

lp-builder-config/misc.yaml

@@ -43,7 +43,8 @@ projects:
           - jammy/edge
       stable/focal:
         channels:
-          - focal/edge
+          # - focal/edge
+          - 8.0.19/edge
 
   - name: MySQL Router
     charmhub: mysql-router
@@ -58,7 +59,8 @@ projects:
           - jammy/edge
       stable/focal:
         channels:
-          - focal/edge
+          # - focal/edge
+          - 8.0.19/edge
 
   - name: Percona Cluster Charm
     charmhub: percona-cluster