SJIP: Clarify that acknowledged issues are not valid

[WangSecurity]

Description

Clarify that issues from previous audits marked as aknowledged and not fixed by the protocol team are considered acceptable risks.

Judging Guidelines PR

https://github.com/sherlock-protocol/sherlock-v2-docs/pull/54/files

Rationale

Currently, our rules only talk about issues marked as won't fix being invalid in update contest. However, when it comes to previous audits outside of Sherlock, we also consider acknowledged issues as known and invalid.

Hence, the goal of this SJIP is to clarify that both issues marked as won't fix in previous contests (if it's an update contest) and marked as acknowledged in previous audits (outside of Sherlock) linked in the contest README are known and invalid.

Relevant Issue Discussions

No specific discussion