keyring: Improve key passphrase parameters

Change Argon2id parameters from 1 gigabyte of memory and 3 iterations
to 2 gigabytes and 2 iterations. Memory cost is the primary parameter
that provides substantial hardening, and 2 GiB in particular is
recommended by RFC 9106 (here with an additional iteration). This will
impact performance of key derivation on devices with constrained RAM,
but platform-backed keys should provide an alternative.

Enforce minimum of 6 characters for the passphrase. This is still not
enough considering an adversary can make unlimited offline guesses,
even with significant slowdown. However, this is consistent with
modern practices for authenticator policies (recommended, for example,
by NIST SP 800-63B-4): enforce minimal length and nothing else.

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