Use CryptoRandomBytes instead of CryptoRandomString (#18439)

- Switch to use `CryptoRandomBytes` instead of `CryptoRandomString`, OAuth's secrets are copied pasted and don't need to avoid dubious characters etc.
- `CryptoRandomBytes` gives ![2^256 = 1.15 * 10^77](https://render.githubusercontent.com/render/math?math=2^256%20=%201.15%20\cdot%2010^77) `CryptoRandomString` gives ![62^44 = 7.33 * 10^78](https://render.githubusercontent.com/render/math?math=62^44%20=%207.33%20\cdot%2010^78) possible states.
- Add a prefix, such that code scanners can easily grep these in source code.
- 32 Bytes + prefix

modules/secret/secret.go

@@ -13,20 +13,8 @@ import (
 	"encoding/hex"
 	"errors"
 	"io"
-
-	"code.gitea.io/gitea/modules/util"
 )
 
-// New creates a new secret
-func New() (string, error) {
-	return NewWithLength(44)
-}
-
-// NewWithLength creates a new secret for a given length
-func NewWithLength(length int64) (string, error) {
-	return util.CryptoRandomString(length)
-}
-
 // AesEncrypt encrypts text and given key with AES.
 func AesEncrypt(key, text []byte) ([]byte, error) {
 	block, err := aes.NewCipher(key)