Soft deprecate ContentHandler::getContentText()

ContentHandler::getContentText() was introduced as a migration
helper and is mostly pointless now. Thus it should be deprecated
and later removed. This patch softs deprecates that method.

This patch also deprecates the corresponding setting
$wgContentHandlerTextFallback.

Bug: T268041
Change-Id: I7d477935a16316996c4f52a3d6466029745817b4

RELEASE-NOTES-1.37

@@ -25,6 +25,9 @@ For notes on 1.36.x and older releases, see HISTORY.
 * …
 
 ==== Changed configuration ====
+* $wgContentHandlerTextFallback - This setting, which defines how to
+  react if a plain text version of a non-text Content object is
+  requested using ContentHandler::getContentText(), is deprecated.
 * …
 
 ==== Removed configuration ====
@@ -295,6 +298,8 @@ because of Phabricator reports.
   ::setHookContainer() were hard deprecated. Use ::init() to inject
   dependencies or override ::postInitSetup() to do any custom
   post-initialization configuration.
+* ContentHandler::getContentText() and is now soft deprecated.
+  Use Content::getText() instead.
 * …
 
 === Other changes in 1.37 ===

includes/DefaultSettings.php

@@ -2420,6 +2420,7 @@ $wgNamespaceContentModels = [];
  * * 'serialize': serialize to default format
  *
  * @since 1.21
+ * @deprecated since 1.37
  */
 $wgContentHandlerTextFallback = 'ignore';
 

includes/content/ContentHandler.php

@@ -81,6 +81,9 @@ abstract class ContentHandler {
 	 *
 	 * @since 1.21
 	 *
+	 * @deprecated since 1.37, use Content::getText() for TextContent instances
+	 * instead
+	 *
 	 * @param Content|null $content
 	 *
 	 * @throws MWException If the content is not an instance of TextContent and