Adding security self-assesment for project K3s

[OrlinVasilev]

Adding the K3s security self-assesment as advised by @eddie-knight https://github.com/cncf/tag-security/pull/1500#discussion_r2597424850

[OrlinVasilev]

@eddie-knight PTAL ^^ Thanks!

[JustinCappos]

FYI: The contents of the assessment were reviewed and approved from the prior PR. https://github.com/cncf/tag-security/pull/1500#pullrequestreview-3521371613