misc: Upgrade Go to v1.21

Upgrade to 1.21 as 1.20 is now unmaintained.

Most changes are related to the new `slices` package from standard
library, which can replace some existing custom functions.

backend/util/util.go

@@ -5,15 +5,6 @@ import (
 	"os"
 )
 
-// Return a slice of range [0, n)
-func Range(n int) []int {
-	s := make([]int, n)
-	for i := 0; i < n; i++ {
-		s[i] = i
-	}
-	return s
-}
-
 func CopyFile(srcPath, dstPath string) error {
 	fin, err := os.Open(srcPath)
 	if err != nil {