Hard-deprecate ParserOutput::preventClickjacking()

Follow up to the soft deprecation in I9822c60c180d204bd30cb4447a1120155d456da4.

Code search:
https://codesearch.wmcloud.org/deployed/?q=%28allow%7Cprevent%29Clickjacking&i=nope&files=&excludeFiles=&repos=

Note that the dependencies here are actually cleanups to
OutputPage::preventClickjacking(), not ParserOutput::preventClickjacking().
It can be hard to tell the difference between these two methods when
reading code (especially when parameter types are omitted) and so we're
being extra cautious by cleaning up both of these together.

Bug: T287216
Depends-On: I626e89a1bff2d9c535b828a5b25eff863c91f858
Depends-On: I23e8b35540d2da34c0a3d335069c518b4b72a333
Depends-On: I2538ac8f37fcde183f496e49a8de0ed25a38e508
Change-Id: I0f9b1fe755f7d3d6a9b18fa21a738dda7566c211

includes/parser/ParserOutput.php

@@ -1419,6 +1419,7 @@ class ParserOutput extends CacheTime {
 	 *   use ::setPreventClickjacking() or ::getPreventClickjacking()
 	 */
 	public function preventClickjacking( $flag ) {
+		wfDeprecated( __METHOD__, '1.38' );
 		$old = $this->getPreventClickjacking();
 		if ( $flag !== null ) {
 			$this->setPreventClickjacking( $flag );