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

e2e/config/host.docker.internal/zitadel.yaml

@@ -9,8 +9,8 @@ Database:
     # This makes the e2e config reusable with an out-of-docker zitadel process and an /etc/hosts entry
     Host:  host.docker.internal
     EventPushConnRatio: 0.2
-    MaxOpenConns: 40
-    MaxIdleConns: 20
+    MaxOpenConns: 15
+    MaxIdleConns: 10
 
 TLS:
   Enabled: false

e2e/config/localhost/zitadel.yaml

@@ -9,8 +9,8 @@ Database:
     # This makes the e2e config reusable with an out-of-docker zitadel process and an /etc/hosts entry
     Host: host.docker.internal
     EventPushConnRatio: 0.2
-    MaxOpenConns: 40
-    MaxIdleConns: 20
+    MaxOpenConns: 15
+    MaxIdleConns: 10
 
 TLS:
   Enabled: false