feat: Make sequoia the default crypto backend - GnuPG becomes "legacy mode" only

The objective of this issue is to have a release of sett for which the usage of GnuPG optional. In other words:

• sett will use sequoia as a cryptographic backend by default, rather than being an "experiemental" feature behind an option.
• For new users (who do not need to migrate a key), sett will be entirely usable without having GnuPG installed.
• The only need for GnuPG will be for users who want to migrate a certificate from the GnuPG keyring to the sett certificate store.

The code related to the current "GnuPG" mode should be removed be moved to "legacy mode".

Do NOT update the documentation as #464 (closed) will take care of it.