[api] Advance deprecation of String::isExternal

V8_DEPRECATE_SOON happened in v8.7

Bug: v8:10641
Change-Id: Ia20f89da38b1e739051d019a395ff1e7989a361b
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2450063Reviewed-by: Victor Gomes <victorgomes@chromium.org>
Commit-Queue: Camillo Bruni <cbruni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70334}

include/v8.h

@@ -3116,7 +3116,7 @@ class V8_EXPORT String : public Name {
    * Returns true if the string is external two-byte.
    *
    */
-  V8_DEPRECATE_SOON(
+  V8_DEPRECATED(
       "Use String::IsExternalTwoByte() or String::IsExternalOneByte()")
   bool IsExternal() const;