Enable dependabot for rust dependency updates

Also increases the frequency for rust-vmm-ci submodules updates to once
a week, and limits the number of simultaneous PRs for it to one. This
brings the configuration in-line with vm-allocator and vm-memory.

Signed-off-by: Patrick Roy <roypat@amazon.co.uk>

Author: roypat

.github/dependabot.yml

@@ -3,5 +3,11 @@ updates:
 - package-ecosystem: gitsubmodule
   directory: "/"
   schedule:
-    interval: monthly
+    interval: weekly
+  open-pull-requests-limit: 1
+- package-ecosystem: cargo
+  directory: "/"
+  schedule: 
+    interval: weekly
   open-pull-requests-limit: 10
+  rebase-strategy: "disabled"