Decrease the frequency of cargo updates

They're development-only dependencies. We shouldn't need to pay the
maintenance cost for such frequent updates, especially when the
dependabot PRs have failed the CI two times in a row.

Author: k0kubun

.github/dependabot.yml

@@ -18,7 +18,7 @@ updates:
     exclude-paths:
       - 'gc/mmtk/**'
     schedule:
-      interval: 'daily'
+      interval: 'monthly'
     groups:
       jit:
         patterns: