Update gem dependencies once a month

We originally set a daily interval because we hadn't updated our gem
dependencies for a year.

However, we usually wait a few days/weeks between the time a gem is
released and the moment we update it, and there are gems releasing new
versions every few days, so maintaining daily updates would become
tedious quickly.

So we're now doing it once a month. We're also increasing the limit of
open pull requests so we don't need to worry about whether dependabot is
opening pull requests for every dependency.

.github/dependabot.yml

@@ -3,7 +3,5 @@ updates:
 - package-ecosystem: bundler
   directory: "/"
   schedule:
-    interval: daily
-    time: "04:00"
-    timezone: Etc/UTC
-  open-pull-requests-limit: 25
+    interval: monthly
+  open-pull-requests-limit: 50