ci: Removes the sync next to main action

[scottlovegrove]

This workflow isn't really all that helpful (even less so now that next has been merged).

It caused a couple of issues when trying to merge next into main because it had created a couple of PRs that had been merged, but they had been merged by squashing, which resulted in next losing history of its own branch.

[doistbot]

This PR removes the automated workflow that syncs the next branch with main to prevent commit history issues caused by squashed merges. While this streamlines the repository's automation, it introduces a couple of inconsistencies in the existing setup. Specifically, the next branch remains active in the release configuration without a mechanism to stay updated, and the removed workflow is still documented in CODEBASE.md.

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