Ensure compatibility of Drupal with PHP 8 (as it evolves)

Postponed on #3086374: Make Drupal 8 compatible with PHP 7.4.

Problem/Motivation

Make sure Drupal keeps being compatible with PHP 8. Work with dependencies as needed. See #3086374: Make Drupal 8 compatible with PHP 7.4 for how we did it with PHP 7.4.

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet