Update Sequoia crypto policy, implement year-ahead warning
Move our cut-offs to February to align with Sequoia, and cut-off more:
- 2024-02: DSA keys retroactively (align with GnuPG config)
- 2026-02: SHA224 hashes
- 2028-02: Brainpool keys (align closer with GnuPG backend)
- 2030-02: RSA2048 keys
These algorithms will not be valid starting on those cut-off dates.
Gbp-Dch: full
Also implement warnings for signatures that will be rejected by the policy within the next year.