Use two new sniffs from PHPCSExtra

Includes raising the minimum supported PHPCSExtra version. (will conflict with PR 2408 and will need rebase depending on which is merged first)

Ref:
* https://github.com/PHPCSStandards/PHPCSExtra/releases/tag/1.2.0

.github/CONTRIBUTING.md

@@ -41,7 +41,7 @@ When you introduce new `public` sniff properties, or your sniff extends a class
 * WordPress-Coding-Standards
 * PHP_CodeSniffer 3.7.2 or higher
 * PHPCSUtils 1.0.8 or higher
-* PHPCSExtra 1.1.0 or higher
+* PHPCSExtra 1.2.0 or higher
 * PHPUnit 4.x, 5.x, 6.x or 7.x
 
 The WordPress Coding Standards use the `PHP_CodeSniffer` native unit test framework for unit testing the sniffs.

WordPress-Core/ruleset.xml

@@ -869,6 +869,9 @@
 	<!-- Important to prevent issues with content being sent before headers. -->
 	<rule ref="Generic.Files.ByteOrderMark"/>
 
+	<!-- Always have a lowertag PHP open tag. -->
+	<rule ref="Universal.PHP.LowercasePHPTag"/>
+
 	<!-- All line endings should be \n. -->
 	<rule ref="Generic.Files.LineEndings">
 		<properties>

WordPress-Extra/ruleset.xml

@@ -180,6 +180,9 @@
 	<!-- Detect useless "echo sprintf(...)". -->
 	<rule ref="Universal.CodeAnalysis.NoEchoSprintf"/>
 
+	<!-- Detect use of double negative `!!`. -->
+	<rule ref="Universal.CodeAnalysis.NoDoubleNegative"/>
+
 	<!--
 	#############################################################################
 	Code style sniffs for more recent PHP features and syntaxes.

composer.json

@@ -23,7 +23,7 @@
 		"ext-xmlreader": "*",
 		"squizlabs/php_codesniffer": "^3.7.2",
 		"phpcsstandards/phpcsutils": "^1.0.8",
-		"phpcsstandards/phpcsextra": "^1.1.0"
+		"phpcsstandards/phpcsextra": "^1.2.0"
 	},
 	"require-dev": {
 		"phpcompatibility/php-compatibility": "^9.0",