No need to clutter checks

Only allow master phpcs and develop wpcs on one PHP version, and that should be allowed to fail

We only need master - master and lowest - lowest for every PHP version, not all 4 possible combinations.

So that it works with the composer v2

Also change the theme review team repo to correct repo name. It's themes team now.

Also, some ideas from PHPCompatibility PR PHPCompatibility/PHPCompatibility#1260 were implemented as well (composer dependencies and xml violations).

... as it would prevent PRs with only markdown changes from being merged due to required statuses.

Triggering a workflow for a branch manually is not supported by default in GH Actions, but has to be explicitly allowed.

Ref: https://github.blog/changelog/2020-07-06-github-actions-manual-triggers-with-workflow_dispatch/

... as it shouldn't be needed.

As things were, the code was only ever linted against PHP 7.4, while in the original Travis workflow, the code was linted against every single supported PHP version.

This adds a hybrid workflow in which the code is linted against the high/low of each PHP major within the range of supported PHP versions.

Not a necessity, but allows for:
1. Faster linting due to the parallel option.
2. The ability to display linting errors in the PR via the cs2pr tooling.

The minimum supported PHPCS version according to the `composer.json` file, is `3.3.1`, so that should be the minimum version to test against.
Similarly, the minimum supported WPCS version according to the `composer.json` file is `2.2.0`.

For PHP 7.4 and 8.0, the builds have to be specified explicitly as PHP 7.4 only has PHPCS runtime support as of PHPCS 3.5.0 and PHP 8.0, as of PHPCS 3.5.7.

As the matrix builds only test high PHPCS with high WPCS and low PHPCS with low WPCS, for PHP 7.2, in the Travis config, explicit builds were specified to reverse that combination, i.e. high PHPCS with low WPCS and low PHPCS with high WPCS.
This has now been implemented here as well.

Includes adding some inline documentation to the matrix.

Note: I've not added a build against PHP 8.1/nightly as the PHPUnit version we are forced to use via PHPCS (PHPUnit 7.x) is not compatible with PHP 8.1, so the build wouldn't pass no matter what.

... based on latest example code generate on the Actions page.