phpunit-mock-objects is marked as "abandoned" when composer.lock is rebuilt

Problem/Motivation

In #3036210: Add justinrainbow/json-schema as a composer dependency so JSON:API can use it to validate its responses, we noticed that phpunit-mock-objects (or at least our current version of it) is being marked as "abandoned" when composer.lock is rebuilt for an unrelated dependency addition.

This is likely due to us using an old version of PHPUnit which has old dependencies.

Proposed resolution

?

Per @larowlan, this probably won't actually break anything, but will probably result in Composer warnings and such that will concern users and lead to bug reports.

The version of phpunit-mock-objects that we use currently is impressively out of date -- 2.3.8, when the current release is 5.0.10. The latest version requires PHP 7.1 (as do the supported versions of PHPUnit itself), but we need to maintain compatibility with PHP 7.0 until #2917655: [9.4.x only] Drop official PHP 7.3 support in Drupal 9.4 is finalized. We also use PHPUnit 4 for testing PHP 5.5 and 5.6, which will be tested once per day on Drupal 8.7.x and 8.6.x following 8.7.0 as per the plan outlined in #2842431: [policy] Remove PHP 5.5, 5.6 support in Drupal 8.7.

Remaining tasks

TBD

User interface changes

N/A

API changes

TBD

Data model changes

N/A

Release notes snippet

TBD