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

internal/eventstore/repository/sql/query.go

@@ -309,7 +309,7 @@ func prepareConditions(criteria querier, query *repository.SearchQuery, useV1 bo
 		}
 
 		for i := range instanceIDs {
-			instanceIDs[i] = dialect.DBPurposeEventPusher.AppName() + "_" + instanceIDs[i]
+			instanceIDs[i] = "zitadel_es_pusher_" + instanceIDs[i]
 		}
 
 		clauses += awaitOpenTransactions(useV1)