firewall.cgi: Fixes XSS potential

firewall.cgi: Fixes XSS potential

- Related to CVE-2025-50975
- Fixes PROT
- ruleremark was already escaped when firewall.cgi was initially merged back in Core
   Update 77.
- SRC_PORT, TGT_PORT, dnaport, src_addr & tgt_addr are already validated in the code as
   ports or port ranges.
- std_net_tgt is a string defined in the code and not a variable
- The variable key ignores any input that is not a digit and subsequently uses the next
   free rulenumber digit

Signed-off-by: Adolf Belka <adolf.belka@ipfire.org>
Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>