all: Modernize error wrapping (#8491)

This replaces old style errors.Wrap with modern fmt.Errorf and removes
the (direct) dependency on github.com/pkg/errors. A couple of cases are
adjusted by hand as previously errors.Wrap(nil, ...) would return nil,
which is not what fmt.Errorf does.

lib/upnp/upnp.go

@@ -37,6 +37,7 @@ import (
 	"bytes"
 	"context"
 	"encoding/xml"
+	"errors"
 	"fmt"
 	"io"
 	"net"
@@ -46,8 +47,6 @@ import (
 	"sync"
 	"time"
 
-	"github.com/pkg/errors"
-
 	"github.com/syncthing/syncthing/lib/build"
 	"github.com/syncthing/syncthing/lib/dialer"
 	"github.com/syncthing/syncthing/lib/nat"