ci(dependabot): drop unsupported semver-* cooldown on github-actions

github-actions honours only default-days; the semver-* keys silently break
parsing for that ecosystem. Matches scribe/raspi-dashboard.

Author: eetu

.github/dependabot.yaml

@@ -38,12 +38,11 @@ updates:
           - "minor"
           - "patch"
 
+  # github-actions only honours `default-days` — the semver-* keys silently
+  # break parsing for this ecosystem, so they're omitted.
   - package-ecosystem: "github-actions"
     directory: "/"
     schedule:
       interval: "weekly"
     cooldown:
       default-days: 7
-      semver-major-days: 14
-      semver-minor-days: 7
-      semver-patch-days: 5