docs: howto_ims.md: include disabling SQN without explanation why this is even needed.

[lynxis]

Feature Request

Summary

https://github.com/omnt/OpenMobileNetworkToolkit/blob/main/docs/howto_ims.md describes how to disable SQN checking. However this is only needed if your core network doesn't contain a common HSS. E.g. in early howtos this was required, because the howto required to have 2 HSS which doesn't share the sequence numbers (e.g. open5gs-hss + pyhss). However it is now possible to use either open5gs-hss or pyhss for both, the IMS authentication and the LTE core network authentication.

It would be great to mention the reason and describe this step doesn't hurt, but is only required if the core network uses a split brain HSS.

[laf0rge]

Any documentation recommending disabling a security feature should also give an explanation of the consequences of doing so, warn against doing it without fully understanding the consequences, and reference related documentation/publications about said security mechanism.

SQN checking protects against replay attacks. Disabling that check consequently exposes you to them.