Build/Test Tools: Remove the Composer lock file from version control.

This makes it easier to run unit tests against multiple different PHP versions.

There is currently no reason to have a `composer.lock` file as:

* External runtime dependencies are not managed via Composer.
* Managed updates of the non-runtime dependencies can be done by locking the version used in the `composer.json` file to a precise version instead of using a `composer.lock` file.
* Having the `composer.lock` file in place makes it a lot more difficult to run the tests against all supported PHP versions.

With these considerations in mind, the lock file is now removed from version control and added to `.gitignore` and `svn:ignore`.

Version constraints for the current dev dependencies are adjusted accordingly:

* PHPUnit now explicitly declares in its version constraints that PHPUnit 5.x, 6.x, and 7.x are supported. The minimum supported version for PHPUnit 5.x has been raised from 5.4 to 5.7, which in practice was already the version used for running the tests on PHP 5.6.
* PHPCompatibilityWP is effectively updated to version 2.1.2 with the positive impact that a few new constants polyfilled in WP 5.8 are now accounted for (excluded from being flagged).
* PHP_CodeSniffer is declared as an explicit dependency to ensure that updates to it will always be explicitly managed instead of inherited.
* The DealerDirect Composer plugin is effectively updated to version 0.7.1 without impact.

Follow-up to [42960], [46290], [47881], [48957].

Props jrf, johnbillion, desrosj, ayeshrajans, aristath, hellofromTonya, SergeyBiryukov.
See #47381.
Built from https://develop.svn.wordpress.org/trunk@51543


git-svn-id: http://core.svn.wordpress.org/trunk@51154 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/version.php

@@ -13,7 +13,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '5.9-alpha-51542';
+$wp_version = '5.9-alpha-51543';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.