Use copy-pr-bot

This PR replaces the `copy_prs` functionality from the `ops-bot` with the new dedicated `copy-pr-bot` GitHub application.

Docs for `copy-pr-bot` can be viewed below.

- https://docs.gha-runners.nvidia.com/apps/copy-pr-bot/

**Important**: `copy-pr-bot` enforces signed commits. If an organization member opens a PR that contains unsigned commits, it will be deemed untrusted and therefore require an `/ok to test` comment. See the GitHub docs [here](https://docs.github.com/en/authentication/managing-commit-signature-verification/about-commit-signature-verification) for information on how to set up commit signing.

Any time a PR is deemed untrusted, it will receive a comment that looks like this: rapidsai/ci-imgs#63 (comment).

.github/copy-pr-bot.yaml

@@ -0,0 +1,4 @@
+# Configuration file for `copy-pr-bot` GitHub App
+# https://docs.gha-runners.nvidia.com/apps/copy-pr-bot/
+
+enabled: true

.github/ops-bot.yaml

@@ -3,4 +3,3 @@
 
 auto_merger: true
 branch_checker: true
-copy_prs: true