Docs might need updating to reflect PSP depreciation

[udf2457]

At present the docs do a good job of explaining how to harden k3s by using PSPs.

However, it appears that PSPs are deprecated as of Kubernetes 1.21

This likely means that the docs should be updated accordingly.

Unfortunate I'm far from a subject-matter expert in this area, so I can't viably propose a PR myself, hence I am just flagging this here.

[cwayne18]

This is a good point and we're aware-- to be fair though they're still usable up until 1.25, so we can't remove the PSP bits while we still support < 1.25, though a fair point we should add PSA docs for 1.25+