Composer: update YoastCS and Composer Installers

[jrfnl]

Summary

This PR can be summarized in the following changelog entry:

• Updated dependencies

Relevant technical choices:

Composer: update YoastCS to v 2.1.0

• Updated YoastCS from 2.0.2 to 2.1.0.
• Updated PHP_CodeSniffer from 3.5.5 to 3.5.8.
• Updated the DealerDirect Composer plugin from 0.6.2 to 0.7.0 (which is compatible with Composer 2.0).

Relevant changes in YoastCS:

• The minimum supported WP version has changed to 5.4.
• A new check for test doubles being named as such.
• A few bugfixes.
• Various sniffs now provide metrics.

Relevant changes in PHPCS:

• PHPCS will now run without problems on PHP 8. Note: it will not necessarily handle all code using PHP 8 syntax correctly yet, though it does contain preliminary support for various syntaxes.
• Lots of bugfixes.

Refs:

• https://github.com/Yoast/yoastcs/releases/tag/2.1.0
• https://github.com/squizlabs/php_codesniffer/releases

Composer: update the Composer installers dependency

Update the Composer Installers dependency from 1.6.0 to 1.9.0 to fix compatibility with Composer 2.0.

Based on the changelog, this shouldn't have any other impact.

Ref: https://github.com/composer/installers/releases

CS: minor tweak

Test instructions

This PR can be tested by following these steps:

• N/A Verify that the build passes

[igorschoester]

I cherry picked this into the hotfix/3.0.1 branch to solve the Travis build problems. Changing the milestone to match.