Remove key creation timestamp from pubkey that is sent out

[Hocuri]

The key creation timestamp is potentially sensitive information which we don't want to send out because there is no need for it. So, it should be set to 0 or the current date or some fixed number.

This may require a new API in rPGP.

This will also make something like #8218 superflous.

cc @link2xt

[link2xt]

We cannot set creation timestamp to 0 for existing keys because creation timestamp goes into the key fingerprint calculation.

We can regenerate key binding signatures when we regenerate the public key to have the same timestamp there as the timestamp of the direct key signature of the public key (currently it is the time of the latest relay change).

As for creation timestamp, we can change it when we create the keys.