PHPUnit: Let PHPUnit Polyfills match PHPUnit version

[hellofromtonya]

Part of #43333

What?

Lets PHPUnit Polyfills match the PHPUnit version to the PHP version.

Before this PR, the Gutenberg tests run on PHP 7.4 and use PHPUnit 6.5. However, the matching PHPUnit version should be PHPUnit 9.

PHPUnit 6.5.14 by Sebastian Bergmann and contributors.

Runtime:       PHP 7.4.[30](https://github.com/WordPress/gutenberg/runs/7878291592?check_suite_focus=true#step:7:31)
Configuration: /var/www/html/wp-content/plugins/gutenberg/phpunit.xml.dist

This PR uses PHPUnit Polyfills to set the PHPUnit version, rather than setting the version in the composer.json file and locking it in the .lock file.

Why?

Reasons:

• To match the PHP and PHPUnit versions.
• To increase parity with WordPress Core's PHP tests.
• To unlock the ability to run Gutenberg PHP tests against all WordPress Core supported PHP versions (for cross-compatibility and parity with Core).
• To reduce backporting efforts.
• PHPUnit 6.5 is not meant for PHP 7.4. There are issues and incompatibilities when running it on PHP 7.4.

How?

See the set up instructions in this Dev Note.

The changes are:

• Remove the Composer dependency from composer.json.
• Remove the composer.lock file. Why? To allow for cross-version PHP testing.
• Exclude committing composer.lock file. Why? One .lock file locks the CI into only those versions specified in that .lock file, thus blocking the ability to do cross-version PHP testing.

CI updates to run cross-version PHP testing is not included in the scope of this PR. A future PR will be coming. Note: WordPress Core runs the PHP test suite on all of the supported versions.

[ockham]

Thank you, @hellofromtonya! LGTM