chore: release_checklist.py checks if 'begin dev cycle' PR is needed

[kim-em]

This PR adds a check to release_checklist.py, to check whether CMakeLists.txt on master has been updated, and if not reminds that a "begin dev cycle" PR (as documented in release_checklist.md is needed.

[leanprover-community-bot]

Mathlib CI status (docs):

• ❗ Batteries/Mathlib CI will not be attempted unless your PR branches off the nightly-with-mathlib branch. Try git rebase 142874d86392450eb91545cdaa831c4b3b87b4d7 --onto 832d7c500d709deb5e7f0a5a6fd0f01865d1a303. (2025-02-03 23:56:31)
• ❗ Batteries/Mathlib CI will not be attempted unless your PR branches off the nightly-with-mathlib branch. Try git rebase 99f514dc5ef5f0cbad9eb89808681acae65df735 --onto 832d7c500d709deb5e7f0a5a6fd0f01865d1a303. (2025-02-04 01:01:50)