ci: tweak the dependabot config for branches a bit

- drop the 1.24 branch for now: changes in the hosted GitHub runners
  [1][2][3] make it hard to test that branch based on EL 7
- demote the update frequency for branches different than "main" to
  monthly rather than weekly, as there is no rush to switch all of them
  to newer versions of actions

[1] https://github.blog/changelog/2024-03-07-github-actions-all-actions-will-run-on-node20-instead-of-node16-by-default/
[2] actions/checkout#1809
[3] actions/runner#3503

Author: ptoscano

.github/dependabot.yml

@@ -11,95 +11,87 @@ updates:
   - package-ecosystem: "github-actions"
     directory: "/"
     schedule:
-      interval: "weekly"
+      interval: "monthly"
     target-branch: "subscription-manager-1.30.6"
     commit-message:
       prefix: "ci"
 
   - package-ecosystem: "github-actions"
     directory: "/"
     schedule:
-      interval: "weekly"
+      interval: "monthly"
     target-branch: "subscription-manager-1.29"
     commit-message:
       prefix: "ci"
 
   - package-ecosystem: "github-actions"
     directory: "/"
     schedule:
-      interval: "weekly"
+      interval: "monthly"
     target-branch: "subscription-manager-1.29.45"
     commit-message:
       prefix: "ci"
 
   - package-ecosystem: "github-actions"
     directory: "/"
     schedule:
-      interval: "weekly"
+      interval: "monthly"
     target-branch: "subscription-manager-1.29.40"
     commit-message:
       prefix: "ci"
 
   - package-ecosystem: "github-actions"
     directory: "/"
     schedule:
-      interval: "weekly"
+      interval: "monthly"
     target-branch: "subscription-manager-1.29.33"
     commit-message:
       prefix: "ci"
 
   - package-ecosystem: "github-actions"
     directory: "/"
     schedule:
-      interval: "weekly"
+      interval: "monthly"
     target-branch: "subscription-manager-1.29.26"
     commit-message:
       prefix: "ci"
 
   - package-ecosystem: "github-actions"
     directory: "/"
     schedule:
-      interval: "weekly"
+      interval: "monthly"
     target-branch: "subscription-manager-1.28"
     commit-message:
       prefix: "ci"
 
   - package-ecosystem: "github-actions"
     directory: "/"
     schedule:
-      interval: "weekly"
+      interval: "monthly"
     target-branch: "subscription-manager-1.28.36"
     commit-message:
       prefix: "ci"
 
   - package-ecosystem: "github-actions"
     directory: "/"
     schedule:
-      interval: "weekly"
+      interval: "monthly"
     target-branch: "subscription-manager-1.28.29"
     commit-message:
       prefix: "ci"
 
   - package-ecosystem: "github-actions"
     directory: "/"
     schedule:
-      interval: "weekly"
+      interval: "monthly"
     target-branch: "subscription-manager-1.28.13"
     commit-message:
       prefix: "ci"
 
   - package-ecosystem: "github-actions"
     directory: "/"
     schedule:
-      interval: "weekly"
+      interval: "monthly"
     target-branch: "subscription-manager-1.26"
     commit-message:
       prefix: "ci"
-
-  - package-ecosystem: "github-actions"
-    directory: "/"
-    schedule:
-      interval: "weekly"
-    target-branch: "subscription-manager-1.24"
-    commit-message:
-      prefix: "ci"