Composer: raise the minimum supported PHPCS version to 3.8.0

... to prevent end-users from running into trouble with the name change.

The files in the Composer `vendor/bin` will only be replaced when the `replace...` directive is found and that is only available in the 3.8.0 tag.
When the files in the Composer `vendor/bin` are not replaced, they will continue to point to the `vendor/squizlabs/php_codesniffer` directory which will no longer exist, leading to fatal "File not found" errors for end-users trying to run PHPCS/PHPCBF.

Includes updating references to the PHPCS version whenever relevant throughout the codebase.

.github/CONTRIBUTING.md

@@ -39,7 +39,7 @@ When you introduce new `public` sniff properties, or your sniff extends a class
 
 ## Pre-requisites
 * WordPress-Coding-Standards
-* PHP_CodeSniffer 3.7.2 or higher
+* PHP_CodeSniffer 3.8.0 or higher
 * PHPCSUtils 1.0.8 or higher
 * PHPCSExtra 1.2.0 or higher
 * PHPUnit 4.x, 5.x, 6.x or 7.x

composer.json

@@ -21,7 +21,7 @@
 		"ext-libxml": "*",
 		"ext-tokenizer": "*",
 		"ext-xmlreader": "*",
-		"phpcsstandards/php_codesniffer": "^3.7.2",
+		"phpcsstandards/php_codesniffer": "^3.8.0",
 		"phpcsstandards/phpcsutils": "^1.0.8",
 		"phpcsstandards/phpcsextra": "^1.2.0"
 	},