phpcs: Clean up after #33287 (#33288)

Changes in #33287 cleaned up our phpcs configs to better ignore rules
that need ignoring where they need ignoring. That means we can get rid
of a lot of `phpcs:ignore` and `phpcs:disable` directives that are no
longer needed.

And while we're at it, let's remove some compatibility code for PHP
before 5.6 and make use of WordPress's polyfills where we can.

Committed via a GitHub action: https://github.com/Automattic/jetpack/actions/runs/6317511559

composer.json

@@ -9,7 +9,7 @@
 		"automattic/jetpack-plans": "^0.3.4",
 		"automattic/jetpack-redirect": "^1.7.27",
 		"automattic/jetpack-status": "^1.18.5",
-		"automattic/jetpack-sync": "^1.57.2"
+		"automattic/jetpack-sync": "^1.57.3-alpha"
 	},
 	"require-dev": {
 		"yoast/phpunit-polyfills": "1.1.0",