ci: reduce update frequency (#113)

Signed-off-by: Shizuo Fujita <fujita@clear-code.com>

Author: Watson1978

.github/dependabot.yml

@@ -3,4 +3,4 @@ updates:
   - package-ecosystem: 'github-actions'
     directory: '/'
     schedule:
-      interval: 'weekly'
+      interval: 'monthly'