chore: fix dependabot config to target default branch and add missing cooldown (#9056)

Remove target-branch pointing to non-existent dependency-updates branch
so dependabot PRs go directly against main. Add missing cooldown settings
to the github-actions ecosystem entry for consistency with npm entry.

Author: just1and0

.github/dependabot.yml

@@ -14,7 +14,6 @@ updates:
       semver-major-days: 7
       semver-minor-days: 7
       semver-patch-days: 7
-    target-branch: dependency-updates
     labels:
       - 'dependencies'
 
@@ -24,6 +23,10 @@ updates:
     directory: '/'
     schedule:
       interval: 'daily'
-    target-branch: dependency-updates
+    cooldown:
+      default-days: 7
+      semver-major-days: 7
+      semver-minor-days: 7
+      semver-patch-days: 7
     labels:
       - 'dependencies'