chore: RL containers no longer accept new pull requests

[jinyoung-lim]

Issue #, if available:

Description of changes: As of 4/2024, SageMaker hosted RL containers will no longer accept pull requests from new customers.

Testing done: N/A

Merge Checklist

Put an x in the boxes that apply. You can also fill these out after creating the PR. If you're unsure about any of them, don't hesitate to ask. We're here to help! This is simply a reminder of what we are going to look for before merging your pull request.

• [x] I have read the CONTRIBUTING doc and adhered to the example notebook best practices
• [x] I have updated any necessary documentation, including READMEs
• [ ] I have tested my notebook(s) and ensured it runs end-to-end
• [x] I have linted my notebook(s) and code using black-nb -l 100 {path}/{notebook-name}.ipynb

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