Revise PHPCS rules relating to PHP compatibility (min: PHP 7.0)

[barryhughes]

Tweaks our PHPCS config so that we allow valid PHP 7.0+ syntax (sometimes, currently, we warn about various constructs that are invalid only when using PHP 5.6 or lower).

Steps to replicate

1. Checkout the current trunk code (ie, without this change) and run PHPCS.
2. If you specifically run composer run phpcs classes/WP_CLI/ActionScheduler_WPCLI_Clean_Command.php you will see various warnings. For the purposes of this PR, we are interested errors such as this one:

  84 | ERROR | [ ] 'int' type declaration is not present in PHP version 5.6 or earlier
     |       |     (PHPCompatibility.FunctionDeclarations.NewParamTypeDeclarations.intFound)

3. This should not be an error: type declarations are desirable and not something we want to remove. The above is flagged because our PHPCS config is looking for code that is incompatible with PHP 5.6, which is no longer a concern for this project.
4. Check out this branch, and re-run the above command: those particular warnings should no longer appear.

Changelog

No changelog needed, this is a dev-facing change.

[barryhughes]

Sorry for the noise re reviewer selection ... had to post some changes to the round-robin rules.

[Konamiman]

Looks good and tests as advertised, so merging.