[cleanup] Move FunctionTemplate::GetFunction to V8_DEPRECATED

Bug: v8:7295, v8:8238
Change-Id: Ide105b69ee77707a5a28d16abfb1ab44ebc3995d
Reviewed-on: https://chromium-review.googlesource.com/c/1352273Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Dan Elphick <delphick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#57895}

include/v8.h

@@ -5739,7 +5739,7 @@ class V8_EXPORT FunctionTemplate : public Template {
       SideEffectType side_effect_type = SideEffectType::kHasSideEffect);
 
   /** Returns the unique function instance in the current execution context.*/
-  V8_DEPRECATE_SOON("Use maybe version", Local<Function> GetFunction());
+  V8_DEPRECATED("Use maybe version", Local<Function> GetFunction());
   V8_WARN_UNUSED_RESULT MaybeLocal<Function> GetFunction(
       Local<Context> context);