FeedItem: Update @since on xmlEncodeNullable for backports

Bug: T385332
Change-Id: I88f82fb5a703126f0ce586030d7ba7666fbb5a18
Follows-Up: Iafd89c0d61baecd7c68f62b2a0764cc78cf25069
(cherry picked from commit 30f076a54660304590c266b7edee949ccc35c237)

includes/Feed/FeedItem.php

@@ -103,7 +103,7 @@ class FeedItem {
 	/**
 	 * Encode $string so that it can be safely embedded in a XML document,
 	 * returning `null` if $string was `null`.
-	 * @since 1.44
+	 * @since 1.44 (also backported to 1.39.12, 1.42.6 and 1.43.1)
 	 */
 	public function xmlEncodeNullable( ?string $string ): ?string {
 		return $string !== null ? $this->xmlEncode( $string ) : null;