Default to session gridifield state manager in CMS 6

[maxime-rainville]

#11255 will add a Ssession Gridfield State manager to CMS 5. This should be the default state manager in CMS 6

Acceptance criteria

• Current GridFieldStateManager is deprecated in CMS 5 and remove in CMS 6.
• Default configuration is updated to use SessionGridFieldStateManager by default in CMS 6
• GridFieldStateManagerInterface is updated to match the signature of SessionGridFieldStateManager
• Solution is abstract enough that a third party developer could create their own GridFieldStateManager by implementing the GridFieldStateManagerInterface in CMS 6
• If feasible, SilverStripe\Forms\GridField\FormAction\SessionStore is removed in favour of the new API.