chore(security): update security.txt with new expiration date (#6655)
Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/6655
Reviewed-by: 0ko <0ko@noreply.codeberg.org>
Reviewed-by: Michael Kriese <michael.kriese@gmx.de>
Reviewed-by: Otto <otto@codeberg.org>