Change npm schedule to monthly (#7033)

The only JavaScript dependencies are in the cirq-web package, which
isn't installed in a public-facing location anywhere. The frequency of
Dependabot updates and PRs has been too high to justify the
maintenance cost.

.github/dependabot.yml

@@ -48,7 +48,7 @@ updates:
     # the specified directory.
     directory: "/cirq-web/cirq_ts/"
     schedule:
-      interval: "weekly"
+      interval: "monthly"
     versioning-strategy: "widen"
     groups:
       # Group non-security version minor & patch updates into one PR.