Mark sapi/embed as non experimental (php#13668)

The embed SAPI has been around for quite a while now, and many apps
already use it in production. It can be marked as non experimental to
avoid confusion.

[skip ci]

Author: petk

sapi/embed/EXPERIMENTAL

@@ -1,6 +1,6 @@
 PHP_ARG_ENABLE([embed],,
   [AS_HELP_STRING([[--enable-embed[=TYPE]]],
-    [EXPERIMENTAL: Enable building of embedded SAPI library TYPE is either
+    [Enable building of embedded SAPI library TYPE is either
     'shared' or 'static'. [TYPE=shared]])],
   [no],
   [no])