functions: Use slices package for sorting

From Go 1.21 release notes[1]:
New slices package for common operations on slices of any element type.
This includes sorting functions that are generally faster and more
ergonomic than the sort package.

[1]: https://go.dev/blog/go1.21

functions.go

@@ -1,6 +1,8 @@
 package main
 
-import "sort"
+import (
+	"slices"
+)
 
 func sortKeys(m map[string]string) []string {
 	var s []string
@@ -8,6 +10,6 @@ func sortKeys(m map[string]string) []string {
 		s = append(s, key)
 	}
 
-	sort.Strings(s)
+	slices.Sort(s)
 	return s
 }