v6.12.0-beta2

Changes since v6.12.0-beta1:

• EZP-28053: As a developer I want search engine capability constants (d3a8b74)
• [Search] Add feature flags for search features to be able to detect new FullText features (#2104)
• Allow FlySystem bundle 2.0 (#2110)
• Remove cannotBeEmpty as it is not applicable to concrete nodes in Symfony 3.4 (#2109)
• [Cache] Variable use when not defined affecting TrashHandler::findTrashItems() without criterion (30b6bc6)
• [Deprecate] CoreBundle kernel class (aceae61)