refactor: use slices package for sort (#10132)

The sort package is still used in places that were not trivial to
change. Since Go 1.21 slices package can be uswed for sort. See
https://go.dev/doc/go1.21#slices

### Purpose

Make some progress with the migration to a more up-to-date syntax.

script/transifexdl.go

@@ -17,7 +17,7 @@ import (
 	"net/http"
 	"os"
 	"regexp"
-	"sort"
+	"slices"
 	"strings"
 )
 
@@ -93,7 +93,7 @@ func main() {
 }
 
 func saveValidLangs(langs []string) {
-	sort.Strings(langs)
+	slices.Sort(langs)
 	fd, err := os.Create("valid-langs.js")
 	if err != nil {
 		log.Fatal(err)

script/weblatedl.go

@@ -17,7 +17,7 @@ import (
 	"net/http"
 	"os"
 	"regexp"
-	"sort"
+	"slices"
 	"strings"
 )
 
@@ -116,7 +116,7 @@ func reformatLanguageCode(origCode string) string {
 }
 
 func saveValidLangs(langs []string) {
-	sort.Strings(langs)
+	slices.Sort(langs)
 	fd, err := os.Create("valid-langs.js")
 	if err != nil {
 		log.Fatal(err)