Configure PHP version for PHP_CodeSniffer

[morozov]

Q	A
Type	improvement
BC Break	no

Currently, phpcs fails with a bunch of false-positive errors if run on PHP 8:

$ php -v
PHP 8.0.8 (cli) (built: Jun 29 2021 22:25:18) ( NTS )
Copyright (c) The PHP Group
Zend Engine v4.0.8, Copyright (c) Zend Technologies
    with Zend OPcache v8.0.8, Copyright (c), by Zend Technologies
    with Xdebug v3.0.4, Copyright (c) 2002-2021, by Derick Rethans

$ phpcs
.......................EE.E......E..........E.........EEE..E 60 / 76 (79%)
E.EEEE...EEEE...                                             76 / 76 (100%)



FILE: tests/Doctrine/Tests/DBAL/Functional/Driver/PDO/ConnectionTest.php
--------------------------------------------------------------------------------------------------------------------------------------------
FOUND 1 ERROR AFFECTING 1 LINE
--------------------------------------------------------------------------------------------------------------------------------------------
 86 | ERROR | [x] Class name referenced via call of function get_class().
    |       |     (SlevomatCodingStandard.Classes.ModernClassNameReference.ClassNameReferencedViaFunctionCall)
--------------------------------------------------------------------------------------------------------------------------------------------
PHPCBF CAN FIX THE 1 MARKED SNIFF VIOLATIONS AUTOMATICALLY
--------------------------------------------------------------------------------------------------------------------------------------------
...
And a lot more.

Note that $object::class is only available as of PHP 8.0. All of the errors are trigger with the assumption that the runtime PHP version (8.0) is the minimum supported version (7.3).

We already have some rules specific to PHP 7.4 suppressed:

dbal/phpcs.xml.dist

Lines 40 to 45 in ce7db44

	<!-- Disable the rules that will require PHP 7.4 -->
	<rule ref="SlevomatCodingStandard.TypeHints.PropertyTypeHint">
	<properties>
	<property name="enableNativeTypeHint" value="false"/>
	</properties>
	</rule>

But there's a better way to configure the minimum supported PHP version.

[morozov]

The "Coding Standards (7.4)" build is irrelevant in the context of this PR. I'll disable it before the merge.