dependabot: check for updates monthly (#620)

Seems very little reason to check more often

.github/dependabot.yml

diff --git a/.github/dependabot.yml b/.github/dependabot.yml
index 430a85e..3f9e337 100644
--- a/.github/dependabot.yml
+++ b/.github/dependabot.yml
@@ -7,5 +7,4 @@
 - package-ecosystem: "github-actions"
   directory: "/"
   schedule:
-    # Check for updates to GitHub Actions every weekday
-    interval: "daily"
+    interval: "monthly"