Deprecate the ParserGetVariableValueVarCache hook

We don't want to expose the internal caching mechanism of the parser.
There are no uses in deployed WMF code:

https://codesearch.wmflabs.org/deployed/?q=ParserGetVariableValueVarCache&i=nope&files=&repos=

Bug: T236813
Change-Id: I1b0a5cf7d32be86a45d7231940360e2bfce7275f

RELEASE-NOTES-1.35

@@ -697,6 +697,7 @@ because of Phabricator reports.
   Use RevisionStore::getFirstRevision instead.
 * The Parser::enableOOUI() method has been deprecated.  Use
   $parser->getOutput()->enableOOUI() instead.
+* The ParserGetVariableValueVarCache hook has been deprecated.
 * …
 
 === Other changes in 1.35 ===

docs/hooks.txt

@@ -2602,8 +2602,8 @@ custom magic word
 [&]$parser: Parser object
 &$time: actual time (timestamp)
 
-'ParserGetVariableValueVarCache': use this to change the value of the variable
-cache or return false to not use it.
+'ParserGetVariableValueVarCache': DEPRECATED since 1.35!  Use this to
+change the value of the variable cache or return false to not use it.
 [&]$parser: Parser object
 &$varCache: variable cache (array)
 

includes/parser/Parser.php

@@ -2694,7 +2694,8 @@ class Parser {
 		 * expensive to check many times.
 		 */
 		if (
-			Hooks::run( 'ParserGetVariableValueVarCache', [ &$parser, &$this->mVarCache ] ) &&
+			// Using this hook to override the magic word cache is deprecated.
+			Hooks::run( 'ParserGetVariableValueVarCache', [ &$parser, &$this->mVarCache ], '1.35' ) &&
 			isset( $this->mVarCache[$index] )
 		) {
 			return $this->mVarCache[$index];