refactor: consolidate database pools (#9105)

# Which Problems Are Solved

Zitadel currently uses 3 database pool, 1 for queries, 1 for pushing
events and 1 for scheduled projection updates. This defeats the purpose
of a connection pool which already handles multiple connections.

During load tests we found that the current structure of connection
pools consumes a lot of database resources. The resource usage dropped
after we reduced the amount of database pools to 1 because existing
connections can be used more efficiently.

# How the Problems Are Solved

Removed logic to handle multiple connection pools and use a single one.

# Additional Changes

none

# Additional Context

part of https://github.com/zitadel/zitadel/issues/8352

cmd/initialise/init.go

@@ -9,7 +9,6 @@ import (
 	"github.com/zitadel/logging"
 
 	"github.com/zitadel/zitadel/internal/database"
-	"github.com/zitadel/zitadel/internal/database/dialect"
 )
 
 var (
@@ -79,7 +78,7 @@ func initialise(ctx context.Context, config database.Config, steps ...func(conte
 		return err
 	}
 
-	db, err := database.Connect(config, true, dialect.DBPurposeQuery)
+	db, err := database.Connect(config, true)
 	if err != nil {
 		return err
 	}