feat(ci): only send (and refresh) dependabot PRs weekly (#3477)

Despite `rebase-strategy: "disabled"`, dependabot will refresh (i.e. rebase) PRs on the specified schedule interval. With the addition of `Cargo.lock` to our repository, dependabot is opening a lot more PRs which consumes unnecessary CI resources.

We change the interval to "weekly" to reduce the noise and resource consumption.

Author: thomaseizinger

.github/dependabot.yml

@@ -3,7 +3,7 @@ updates:
   - package-ecosystem: "cargo"
     directory: "/"
     schedule:
-      interval: "daily"
+      interval: "weekly"
     open-pull-requests-limit: 9999
     commit-message:
       prefix: "deps"