This PR starts by splitting AbstractDoctrineMiddleware into AbstractOrmMiddleware and AbstractDbalMiddleware.
The latter deprecates but still allows to be passed an entity manager name. A connection name can be passed when also passing $isConnectionName to true, which makes the change opt-in.

Then for Symfony 9 we can

• remove AbstractDbalMiddleware support for entity managers
• replace AbstractDbalMiddleware::$managerRegistry by a ConnectionRegistry
• rename AbstractDbalMiddleware::$managerOrConnectionName $connectionName
• deprecate passing AbstractDbalMiddleware::$isConnectionName (since it will then be unused)

Middlewares were updated accordingly by extending AbstractDbalMiddleware:

• DoctrineCloseConnectionMiddleware and DoctrineOpenTransactionLoggerMiddleware were no-brainers since they only act on connections (see [DoctrineBridge] Cannot set doctrine_open_transaction_logger’s entity manager name the same way than other middlewares #64194 though)
• DoctrinePingConnectionMiddleware still resets closed entity managers if $isConnectionName is false to keep BC, but a new DoctrineResetClosedEntityManagerMiddleware was created for this use-case (will need to be wired by the DoctrineBundle).
  I’m not sure this is the right call from a DX perspective though, which is why this PR doesn’t update the DoctrineTransactionMiddleware: should we rather make those act on entity managers depending on $isConnectionName? Or should we split them into an ORM and DBAL parts?