Add instructions to sync release-next with main at code freeze

[thomashampson]

Description of work

Since the Docker container for deployment-tools disappeared, the Jenkins job for pointing release-next to the tip of main wasn't working. After discussion, we decided the best plan was to simply add instructions to the release checklist for performing the task manually. Otherwise it is too easy for any developer to click the "run job" button and spoil the release-next branch part way through the release period.

Fixes #36918

To test:

Check that the instructions are clear and the logic is correct.

This does not require release notes because it's an update to dev docs

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[robertapplin]

Looks good, easy to follow!