fs: remove experimental warning for fs.promises

This has been warning for long enough, without any API changes
in the last few months.

PR-URL: https://github.com/nodejs/node/pull/26581


Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Anto Aravinth <anto.aravinth.cse@gmail.com>
Reviewed-By: Yongsheng Zhang <zyszys98@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Signed-off-by: Beth Griggs <Bethany.Griggs@uk.ibm.com>