Revert "Decrease frequency of GitHub Actions update checks"

This reverts commit 6036cb9.

Since taiki-e/install-action is no longer used, a weekly check shouldn't
be a burden, and I'd rather have all the dependency checks happen at
a consistent interval.

Author: Ortham

.github/dependabot.yml

@@ -8,4 +8,4 @@ updates:
 - package-ecosystem: github-actions
   directory: "/"
   schedule:
-    interval: monthly
+    interval: weekly