ci: tighten size threshold to 1% or 500 bytes (#33969)

The size diff threshold of 1% has proven to be too lenient for us
to catch size regressions in AIO. Since the AIO main bundle is
between 400-500 KB, a size regression must be between 4-5 KB before
it will cause the tests to fail. As a result, we may merge many
changes with smaller regressions of a few KB before the size test
eventually lets us know that the number has increased. The hope is
that lowering the threshold will help us catch the smaller
regressions during code review and prevent the size tests failing at
a random later time when someone catches the size "hot potato".

PR Close #33969

aio/scripts/_payload-limits.json

@@ -21,7 +21,7 @@
     "master": {
       "uncompressed": {
         "runtime-es2015": 3097,
-        "main-es2015": 436779,
+        "main-es2015": 439352,
         "polyfills-es2015": 52503
       }
     }