[api] Deprecate legacy RegisterExtension method

The new API receives a unique_ptr to avoid leaking the Extension object.
All chromium uses were refactored in https://crrev.com/c/1447652.

R=adamk@chromium.org

Bug: v8:8725
Cq-Include-Trybots: luci.chromium.try:linux-blink-rel
Change-Id: I46a931a73e941fe7b78f5390fec74663677e13e4
Reviewed-on: https://chromium-review.googlesource.com/c/1454723Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59451}

include/v8.h

@@ -6502,7 +6502,7 @@ class V8_EXPORT Extension {  // NOLINT
   bool auto_enable_;
 };
 
-V8_DEPRECATE_SOON(
+V8_DEPRECATED(
     "Use unique_ptr version or stop using extension (http://crbug.com/334679).",
     void V8_EXPORT RegisterExtension(Extension* extension));