ci: disable dependabot cargo updates

Until we have a better sense of how to handle the update PRs it produces
let's turn this off for now. I think I was premature in enabling it.

.github/dependabot.yml

@@ -1,14 +1,5 @@
 version: 2
 updates:
-- package-ecosystem: cargo
-  directory: "/"
-  schedule:
-    interval: weekly
-  open-pull-requests-limit: 5
-  groups:
-    crates-io:
-      patterns:
-        - "*"
 - package-ecosystem: github-actions
   directory: "/"
   schedule: