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/eventstore/handler/v2/field_handler.go

@@ -10,7 +10,6 @@ import (
 	"github.com/jackc/pgx/v5/pgconn"
 
 	"github.com/zitadel/zitadel/internal/eventstore"
-	"github.com/zitadel/zitadel/internal/telemetry/tracing"
 )
 
 type FieldHandler struct {
@@ -98,9 +97,7 @@ func (h *FieldHandler) processEvents(ctx context.Context, config *triggerConfig)
 		defer cancel()
 	}
 
-	ctx, spanBeginTx := tracing.NewNamedSpan(ctx, "db.BeginTx")
 	tx, err := h.client.BeginTx(txCtx, nil)
-	spanBeginTx.EndWithError(err)
 	if err != nil {
 		return false, err
 	}

internal/eventstore/handler/v2/handler.go

@@ -20,7 +20,6 @@ import (
 	"github.com/zitadel/zitadel/internal/migration"
 	"github.com/zitadel/zitadel/internal/repository/instance"
 	"github.com/zitadel/zitadel/internal/repository/pseudo"
-	"github.com/zitadel/zitadel/internal/telemetry/tracing"
 )
 
 type EventStore interface {
@@ -477,9 +476,7 @@ func (h *Handler) processEvents(ctx context.Context, config *triggerConfig) (add
 		defer cancel()
 	}
 
-	ctx, spanBeginTx := tracing.NewNamedSpan(ctx, "db.BeginTx")
 	tx, err := h.client.BeginTx(txCtx, nil)
-	spanBeginTx.EndWithError(err)
 	if err != nil {
 		return false, err
 	}