4.0.0: Persistence, PHP 7.4+

• Merged PR #16: Add web.session.Persistence base class superseding the
  ISession interface, which becomes deprecated.
  (@thekid)
• Heads up: Dropped support for PHP < 7.4, see xp-framework/rfc#343
  (@thekid)
• Added PHP 8.5 and 8.6 to test matrix - @thekid