Use copy-pr-bot (#360)

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