refactor(database): exchange connection pool (#8325)

# Which Problems Are Solved

The connection pool of go uses a high amount of database connections.

# How the Problems Are Solved

The standard lib connection pool was replaced by `pgxpool.Pool`

# Additional Changes

The `db.BeginTx`-spans are removed because they cause to much noise in
the traces.

# Additional Context

- part of https://github.com/zitadel/zitadel/issues/7639

internal/integration/config/postgres.yaml

@@ -3,8 +3,7 @@ Database:
     Host: localhost
     Port: 5432
     Database: zitadel
-    EventPushConnRatio: 0.2
-    MaxOpenConns: 40
+    MaxOpenConns: 15
     MaxIdleConns: 10
     User:
       Username: zitadel