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/database/database.go

@@ -14,7 +14,6 @@ import (
 	_ "github.com/zitadel/zitadel/internal/database/cockroach"
 	"github.com/zitadel/zitadel/internal/database/dialect"
 	_ "github.com/zitadel/zitadel/internal/database/postgres"
-	"github.com/zitadel/zitadel/internal/telemetry/tracing"
 	"github.com/zitadel/zitadel/internal/zerrors"
 )
 
@@ -39,9 +38,7 @@ func (db *DB) Query(scan func(*sql.Rows) error, query string, args ...any) error
 }
 
 func (db *DB) QueryContext(ctx context.Context, scan func(rows *sql.Rows) error, query string, args ...any) (err error) {
-	ctx, spanBeginTx := tracing.NewNamedSpan(ctx, "db.BeginTx")
 	tx, err := db.BeginTx(ctx, &sql.TxOptions{ReadOnly: true, Isolation: sql.LevelReadCommitted})
-	spanBeginTx.EndWithError(err)
 	if err != nil {
 		return err
 	}
@@ -74,9 +71,7 @@ func (db *DB) QueryRow(scan func(*sql.Row) error, query string, args ...any) (er
 }
 
 func (db *DB) QueryRowContext(ctx context.Context, scan func(row *sql.Row) error, query string, args ...any) (err error) {
-	ctx, spanBeginTx := tracing.NewNamedSpan(ctx, "db.BeginTx")
 	tx, err := db.BeginTx(ctx, &sql.TxOptions{ReadOnly: true, Isolation: sql.LevelReadCommitted})
-	spanBeginTx.EndWithError(err)
 	if err != nil {
 		return err
 	}