Replace fmt.Sprintf with hex.EncodeToString (#21960)

`hex.EncodeToString` has better performance than `fmt.Sprintf("%x",
[]byte)`, we should use it as much as possible.

I'm not an extreme fan of performance, so I think there are some
exceptions:

- `fmt.Sprintf("%x", func(...)[N]byte())`
- We can't slice the function return value directly, and it's not worth
adding lines.
    ```diff
    func A()[20]byte { ... }
    - a := fmt.Sprintf("%x", A())
    - a := hex.EncodeToString(A()[:]) // invalid
    + tmp := A()
    + a := hex.EncodeToString(tmp[:])
    ```
- `fmt.Sprintf("%X", []byte)`
- `strings.ToUpper(hex.EncodeToString(bytes))` has even worse
performance.

models/migrations/base/hash.go

@@ -5,12 +5,12 @@ package base
 
 import (
 	"crypto/sha256"
-	"fmt"
+	"encoding/hex"
 
 	"golang.org/x/crypto/pbkdf2"
 )
 
 func HashToken(token, salt string) string {
 	tempHash := pbkdf2.Key([]byte(token), []byte(salt), 10000, 50, sha256.New)
-	return fmt.Sprintf("%x", tempHash)
+	return hex.EncodeToString(tempHash)
 }