Hard deprecate ParserOutput::addWarning()

Code search:
https://codesearch.wmcloud.org/deployed/?q=%5BOo%5Dutput%28%5C%28%5C%29%29%3F-%3EaddWarning&i=nope&files=&excludeFiles=&repos=

Other uses in non-production code:
Id96ba92b46ae57a385b7096a02a63ad5eddf566e (BlueSpiceBookshelf)
I8eb6733d676a56f6a03b9798d3c4022493c01d04 (PageInCat)
I9cb8d36caf4326b3c3d02362cd06f2d4093b5a8e (TEI)

Bug: T293515
Depends-On: I352d161dc276c6684b0532b62c0e676a4d05cdfd
Depends-On: Ib51c75e38b9bf490b4ded481ee92ede0949f272e
Change-Id: I99cfbfbecf134366d1a97e67c46d0abc0af22d26

RELEASE-NOTES-1.38

@@ -226,11 +226,11 @@ because of Phabricator reports.
    - ::setCategoryLinks()    - use ::setCategories()
    - ::addTrackingCategory() - use Parser::addTrackingCategory()
                                or TrackingCategories::addTrackingCategory()
+   - ::addWarning()          - use ::addWarningMsg()
 * The following methods from the ParserOutput class were deprecated:
    - ::getFlag() - use ::getOutputFlag()
    - ::setFlag() - use ::setOutputFlag()
    - ::getAllFlags() - this method is now marked @internal
-   - ::addWarning() - use ::addWarningMsg()
 * The following methods were deprecated; use ::setPreventClickjacking(..)
   instead:
    - OutputPage::preventClickjacking()

includes/parser/ParserOutput.php

@@ -842,6 +842,7 @@ class ParserOutput extends CacheTime {
 	 * @deprecated since 1.38; use ::addWarningMsg() instead
 	 */
 	public function addWarning( $s ) {
+		wfDeprecated( __METHOD__, '1.38' );
 		$this->mWarnings[$s] = 1;
 	}