oss: attempt to disable dependabot PRs

Summary:
The dependabot PRs for Javascript and Rust cannot be used as-is because our
monorepo requires "offline mirrors" for dependencies that dependabot cannot
directly update. So let's just limit the dependabot PR to 0. I guess this
does not disable security alerts which are shown separately
(https://github.com/facebook/sapling/security/dependabot).

Reviewed By: zzl0

Differential Revision: D54129988

fbshipit-source-id: b98cd903786b4a0245a17c0343733b22a1f000c6

Author: quark-zju

dependabot.yml

@@ -0,0 +1,14 @@
+# https://docs.github.com/en/code-security/dependabot/dependabot-version-updates/configuration-options-for-the-dependabot.yml-file
+
+version: 2
+updates:
+  - package-ecosystem: "npm"
+    directory: "/"
+    schedule:
+      interval: "weekly"
+    open-pull-requests-limit: 0
+  - package-ecosystem: "cargo"
+    directory: "/"
+    schedule:
+      interval: "weekly"
+    open-pull-requests-limit: 0