Drop support for CentOS 7

charmcraft.yaml

@@ -13,9 +13,6 @@ bases:
       - name: ubuntu
         channel: "20.04"
         architectures: [amd64]
-      - name: centos
-        channel: "7"
-        architectures: [amd64]
 
 parts:
   charm:

config.yaml

@@ -8,9 +8,7 @@ options:
       This can be set to the Organization's local mirror/cache of packages and
       supersedes the Omnivector repositories. Alternatively, it can be used to
       track a `testing` Slurm version, e.g. by setting to
-      `ppa:omnivector/osd-testing` (on Ubuntu), or
-      `https://omnivector-solutions.github.io/repo/centos7/stable/$basearch`
-      (on CentOS).
+      `ppa:omnivector/osd-testing`.
 
       Note: The configuration `custom-slurm-repo` must be set *before*
       deploying the units. Changing this value after deploying the units will