ci: fix dependabot.yml schema validation

As specified in the documentation move prefix under commit-message
section.

Link: <https://docs.github.com/en/code-security/dependabot/working-with-dependabot/dependabot-options-reference>
Signed-off-by: Iker Pedrosa <ipedrosa@redhat.com>
Reviewed-by: Justin Stephenson <jstephen@redhat.com>
Reviewed-by: Pavel Březina <pbrezina@redhat.com>

Author: ikerexxe

.github/dependabot.yml

@@ -2,7 +2,8 @@
 version: 2
 updates:
   - package-ecosystem: github-actions
-    prefix: ci
     directory: /
     schedule:
       interval: monthly
+    commit-message:
+      prefix: ci