Initial proposal for RSA key length wording

What does this MR do?

Follows up Document max RSA key size change (!129191 - merged) to start revising the info in doc/user/ssh.md about RSA key length limitations imposed by Go. That MR needed to go out quickly, so I opted to approve the docs language as-is and revise in a separate MR.

This page belongs to ~"group::authentication and authorization", so it's not one of the usual ones I work on:

• @ashmckenzie, I'm adding you as a reviewer thanks to your information in !129191 (comment 1514634733). Pass this MR forward to @jglassman1 with your changes; he's the TW for ~"group::authentication and authorization".
• @jglassman1, tbh I'm muddling around here and don't feel great about my words here. My limited hours mean you get to see a gnarly draft. 😁 There's also discussion in !129191 (merged) about whether or not to do a release post item for this change. Would that be handled by you or me?

Related issues

• Related to Golang 1.20.7+ RSA key 8192 bit limitation (&11186 - closed)

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