Web: Bump closure compiler spec to ECMASCRIPT_2021

Fixes #88008.

(cherry picked from commit d29b0d90e0d735eee860b977c57cb75423d7a9ba)

platform/javascript/detect.py

@@ -151,7 +151,7 @@ def configure(env):
     env.AddMethod(create_template_zip, "CreateTemplateZip")
 
     # Closure compiler extern and support for ecmascript specs (const, let, etc).
-    env["ENV"]["EMCC_CLOSURE_ARGS"] = "--language_in ECMASCRIPT6"
+    env["ENV"]["EMCC_CLOSURE_ARGS"] = "--language_in ECMASCRIPT_2021"
 
     env["CC"] = "emcc"
     env["CXX"] = "em++"