Deprecate ParserOutput::setLanguageLinks()

Core does not use ::setLanguageLinks() at all any more, and only a very
small number of extensions use it:

  https://codesearch.wmcloud.org/search/?q=-%3EsetLanguageLinks&files=&excludeFiles=&repos=

This can be deprecated and (eventually) removed as we prepare to update
::addLanguageLink().

Bug: T296019
Change-Id: I5bcbc65c8dccb6e3037f528bd9e7e9e27514ea5b

RELEASE-NOTES-1.42

@@ -108,6 +108,7 @@ because of Phabricator reports.
   Vuex 4 will remain accessible for the foreseeable future. Pinia should be used
   for new projects.
 * Title::getBrokenLinksFrom() has been deprecated.
+* ParserOutput::setLanguageLinks() has been deprecated.
 * …
 
 === Other changes in 1.42 ===

includes/parser/ParserOutput.php

@@ -810,6 +810,9 @@ class ParserOutput extends CacheTime implements ContentMetadataCollector {
 		return wfSetVar( $this->mText, $text, true );
 	}
 
+	/**
+	 * @deprecated since 1.42, use ::addLanguageLink() instead.
+	 */
 	public function setLanguageLinks( $ll ) {
 		return wfSetVar( $this->mLanguageLinks, $ll );
 	}