fs: mark recursive cp methods as stable

PR-URL: #53127
Fixes: #44598
Reviewed-By: Antoine du Hamel <[email protected]>
Reviewed-By: Luigi Pinca <[email protected]>
Reviewed-By: James M Snell <[email protected]>
Reviewed-By: Mohammed Keyvanzadeh <[email protected]>

Author: theoludwig