3.0.0: Session layout

This major release drops support for old session layouts with keys in the document root. Sessions with these will be regarded invalid, users
will need to re-authenticate.

• Heads up: Dropped support for PHP < 7.4, see xp-framework/rfc#343
  (@thekid)
• Added support for MongoDB v3 - connecting lazily by default - @thekid
• Merged PR #9: Drop support for MongoDB v1 - @thekid