Add support for Drupal 10; drop support for Drupal 8

Problem/Motivation

This module has test failures in Drupal 10 due to new upstream deprecation. It also relies on a system test fixture that has been removed in 10.x.

1. Increase minimum required Drupal 9 version to ^9.3 to rely on the 9.3.0 upgrade path fixture
2. Resolve various PHPUnit deprecations

Proposed resolution

Resolve D10 compatibility issues and declare compatibility with Drupal 10.

Remaining tasks

Patch, review, commit.

User interface changes

None

API changes

None

Data model changes

None