Drop support of PHPUnit 6 in Drupal 9 because it will never get used anyway

$ COMPOSER_ROOT_VERSION=9.0.x-dev composer update phpunit/phpunit phpspec/prophecy --with-dependencies
...
$ composer-lock-diff --no-links
+-----------------------------------+--------+---------+
| Dev Changes                       | From   | To      |
+-----------------------------------+--------+---------+
| myclabs/deep-copy                 | 1.7.0  | 1.9.5   |
| phar-io/manifest                  | 1.0.1  | 1.0.3   |
| phar-io/version                   | 1.0.1  | 2.0.1   |
| phpdocumentor/reflection-common   | 1.0.1  | 2.0.0   |
| phpdocumentor/reflection-docblock | 4.3.2  | 4.3.4   |
| phpdocumentor/type-resolver       | 0.5.1  | 1.0.1   |
| phpspec/prophecy                  | 1.9.0  | v1.10.2 |
| phpunit/php-code-coverage         | 5.3.2  | 6.1.4   |
| phpunit/php-file-iterator         | 1.4.5  | 2.0.2   |
| phpunit/php-timer                 | 1.0.9  | 2.1.2   |
| phpunit/php-token-stream          | 2.0.2  | 3.1.1   |
| phpunit/phpunit                   | 6.5.14 | 7.5.20  |
| phpunit/phpunit-mock-objects      | 5.0.10 | REMOVED |
| sebastian/comparator              | 2.1.3  | 3.0.2   |
| sebastian/diff                    | 2.0.1  | 3.0.2   |
| sebastian/environment             | 3.1.0  | 4.2.3   |
| sebastian/resource-operations     | 1.0.0  | 2.0.1   |
| webmozart/assert                  | 1.5.0  | 1.6.0   |
+-----------------------------------+--------+---------+

Rebased against 9.0.x.

Reparenting this as the old parent is now closed.

Tagging as a requirement for Drupal 9.0-beta1.

What is still left here? No steps listed in the summary. The patch looks good to me but have not enough experience for sure to confidently say its RTBC :)

There is a significant overlap with #3109261: Update composer dependencies on 9.0.x following PHP 7.3 requirement and this is one of two remaining backend dependency issues for the beta. Woot :)

Updated IS. This does overlap, as the patch here was created before that other issue; this one only upgrades PHPUnit and immediate dependencies but also removes the PHPUnit 6 compatibility layer. Technically we could upgrade without removing the layer but it would leave dead code behind. Therefore both issues/patches are needed but one will require reroll depending on which gets in first :)

Makes sense. We can do this first :) What is still left here other than reviews?

Shouldn't we be going to PHPUnit 8?

@xjm We surely will eventually but that requires at least #3107732: Add return typehints to setUp/tearDown methods in concrete test classes first which is a huge BC break, and probably some other fixes on top. I think this can be moved forward in parallel, especially as PHPUnit 6 isn't even used any more; this is all dead code.

The PHPUnit 8 support issue is at #3063887: Support PHPUnit 8 in Drupal 9, drop support for PHPUnit 7 btw :)

Committed 53b4bf9 and pushed to 9.0.x. Thanks!

If we need to re-implement so of this to support future versions of PHPUnit so be it. We'll do that as necessary. I think we have way more important things to do than to try to backport PHPUnit 8 to Drupal 8 and this nicely unblocks Drupal 9 from moving to PHPUnit 8.

Should we have not removed

        "drupal-phpunit-upgrade-check": "Drupal\\Core\\Composer\\Composer::upgradePHPUnit",
        "drupal-phpunit-upgrade": "@composer update phpunit/phpunit symfony/phpunit-bridge phpspec/prophecy symfony/yaml --with-dependencies --no-progress",

as well?