D9, PHP 7.2 vendor min Testing issue - DO NOT COMMIT

Related issue for Drupal 8.8/8.9: #3044175: PHP7.1 vendor min Testing issue - DO NOT COMMIT

Problem/Motivation

Since testbot only tests issues with the vendor versions in the composer lock file, we need a way to test against older vendor versions that are permitted in our composer.json in order to avoid committing code that may not work with an earlier dependency.

Proposed resolution

Keep an RTBC issue with a patch to run composer update --prefer-lowest against the codebase prior to running the test suite.

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet