[META] Various updates when PHP 8.0 will become required

Problem/Motivation

This issue to collect all todos and updates discovered while making core compatible with PHP 8.0 in #3109885: [meta] Ensure compatibility of Drupal 9 with PHP 8.0 (as it evolves)

Add where all upcoming clean-ups and related issues

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet