Draft: Add expiration date for algorithms in v5 keys

As discussed at the OpenPGP summit this PR is a first attempt at ensuring algorithm freshness. The proposed dates have been picked to ensure that implementations are kept updated, as well as reflecting a (personal) opinion on the current level of confidence we have in the algorithms.

If this proposal is to be accepted it would pave the road to a future algorithm clean-up, and the same could be applied to PQC compsite algorithms.