contrib/check-config: check SCTP protocol option

[randomnoise]

• closes CONFIG_IP_SCTP, INET_DIAG as implicit dependencies #50895

- What I did

Added the Stream Control Transmission Protocol (SCTP) Linux kernel config option to 'Optional Features' in contrib/check-config.sh alphabetically.

I added to 'Optional Features' because when the SCTP config option is disabled, Docker is not broken in any major way, and also SCTP's config explanation ends with this line:

CONFIG_IP_SCTP:
...
If in doubt, say N. (exclude)

It seemed more suitable there.

- How to verify it

Run contrib/check-config.sh against host's kernel config:

$ contrib/check-config.sh config-6.16.5-custom | grep IP_SCTP
- CONFIG_IP_SCTP: missing

$ contrib/check-config.sh config-6.14.0-29-generic | grep IP_SCTP
- CONFIG_IP_SCTP: enabled (as module)

- A picture of a cute animal (not mandatory but encouraged)

[akerouanton]

LGTM. Thanks!