[v8] Make relogin attempts use the strongest auth method (#11781)

[codingllama]

Fixes a potential stdin hijacking bug by making relogin attempts default to a
single MFA method (the strongest available).

The problematic scenario is as follows:

1. User has both OTP and security keys registered
2. "Relogin" is triggered via a tsh command (say,
   tsh logout; tsh ssh --proxy=example.com llama@myserver)
3. User is prompted to pick either OTP or security key ("Tap any security key or
   enter a code from a OTP device")
4. An stdin read is fired in the background to read the OTP code (via
   prompt.Stdin)
5. User picks the security method, thus the stdin read is "abandoned"

In most cases this is fine, as the program ends right after. The issue is when a
relogin is triggered by a long living tsh invocation (again, tsh ssh ...): in
this case the stdin hijack causes input to be swallowed.

Forcing a single MFA option avoids the potential stdin hijack, fixing the
problem for all relogin invocations. tsh login behavior remains the same.

Note that we have to default to cluster's most secure method without checking
the user devices. The user is not logged in yet, thus the backend cannot reveal
any information about that user.

Issue #11709.

[codingllama]

Similar to #11847 - doesn't backport cleanly, so changes are in the second commit.