Doc: update RELEASE_CHANGES checklist.
Update checklist to reflect current practice:
* The platform-specific FAQ files are long gone.
* We've never routinely updated the libbind code we borrowed, either,
and there seems no reason to start now.
* Explain current practice of running pgindent twice per cycle.
Discussion: https://postgr.es/m/
4038398.
1620238684@sss.pgh.pa.us