Use copy-pr-bot (#455)

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

Thorough documentation for the new `copy-pr-bot` application 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).

Every subsequent commit on an untrusted PR will require an additional `/ok to test` comment.

Any existing PRs that have unsigned commits after this change is merged will require an `/ok to test` comment for each subsequent commit _or_ the PR can be rebased to include signed commits as mentioned in the docs below:
https://docs.gha-runners.nvidia.com/cpr/contributors.

This information is all included on the documentation page linked above.

_I've skipped CI on this PR since it's not a change that is tested._

[skip ci]

.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

@@ -5,5 +5,4 @@ auto_merger: true
 branch_checker: true
 label_checker: true
 release_drafter: true
-copy_prs: true
 external_contributors: false