YoastCS rules: disallow the "mixed" type in annotations

An exception is made for the tests, as a data provider for input validation within a function could well be passing every single different type, in which case, `mixed` is perfectly valid.

Related to 303

Fixes 196

Impact on Yoast packages:

| Plugin/Tool       | Errors/Warnings |
|-------------------|-----------------|
| PHPUnit Polyfills | 49 (this is valid though as these are assertion declarations)
| WP Test Utils     | --
| YoastCS           | 2
| WHIP              | --
| Yoast Test Helper | 4
| Duplicate Post    | 12
| Yst ACF           | 1
| Yst WooCommerce   | 2
| Yst News          | 1
| Yst Local         | 42
| Yst Video         | 12
| Yst Premium       | 17
| Yst Free          | 151

Yoast/Reports/Threshold.php

@@ -21,6 +21,7 @@
  * @since 2.2.0
  *
  * @phpcs:disable VariableAnalysis.CodeAnalysis.VariableAnalysis.UnusedVariable -- Flags unused params which are required via the interface. Invalid.
+ * @phpcs:disable SlevomatCodingStandard.TypeHints.DisallowMixedTypeHint.DisallowedMixedTypeHint -- Type is too complex to document properly.
  */
 final class Threshold implements Report {
 

Yoast/ruleset.xml

@@ -387,6 +387,12 @@
 		<exclude name="SlevomatCodingStandard.TypeHints.ReturnTypeHint.UselessAnnotation"/>
 	</rule>
 
+	<!-- Disallows usage of "mixed" type hint. -->
+	<rule ref="SlevomatCodingStandard.TypeHints.DisallowMixedTypeHint">
+		<!-- Make an exception for the tests as "mixed" is a valid type for testing type validation in code. -->
+		<exclude-pattern>*/tests/*\.php$</exclude-pattern>
+	</rule>
+
 	<!-- CS: don't allow "// end class" comments and the likes. -->
 	<rule ref="PSR12.Classes.ClosingBrace"/>