[LINT] Add deadcode linter

- Add the experimental
[deacode](https://pkg.go.dev/golang.org/x/tools/internal/cmd/deadcode)
linter to Forgejo.
- To deal with false positives that can happen due to build tags or with code
that's currently only referenced by test code, the output of the tool is
compared against a known-good output.
- This commit doesn't make any attempt to remove any deadcode.

(cherry picked from commit ac462279e9)
(cherry picked from commit b5ea6e85ac)
(cherry picked from commit 5915f3643c)

[CLEANUP] Remove deadcode

- This is deadcode since https://codeberg.org/forgejo/forgejo/pulls/1802
removed the usage of it.

(cherry picked from commit d840b9923e)
(cherry picked from commit 9442bab626)

modules/util/legacy.go

@@ -4,10 +4,6 @@
 package util
 
 import (
-	"crypto/aes"
-	"crypto/cipher"
-	"crypto/rand"
-	"errors"
 	"io"
 	"os"
 )
@@ -40,52 +36,3 @@ func CopyFile(src, dest string) error {
 	}
 	return os.Chmod(dest, si.Mode())
 }
-
-// AESGCMEncrypt (from legacy package): encrypts plaintext with the given key using AES in GCM mode. should be replaced.
-func AESGCMEncrypt(key, plaintext []byte) ([]byte, error) {
-	block, err := aes.NewCipher(key)
-	if err != nil {
-		return nil, err
-	}
-
-	gcm, err := cipher.NewGCM(block)
-	if err != nil {
-		return nil, err
-	}
-
-	nonce := make([]byte, gcm.NonceSize())
-	if _, err := rand.Read(nonce); err != nil {
-		return nil, err
-	}
-
-	ciphertext := gcm.Seal(nil, nonce, plaintext, nil)
-	return append(nonce, ciphertext...), nil
-}
-
-// AESGCMDecrypt (from legacy package): decrypts ciphertext with the given key using AES in GCM mode. should be replaced.
-func AESGCMDecrypt(key, ciphertext []byte) ([]byte, error) {
-	block, err := aes.NewCipher(key)
-	if err != nil {
-		return nil, err
-	}
-
-	gcm, err := cipher.NewGCM(block)
-	if err != nil {
-		return nil, err
-	}
-
-	size := gcm.NonceSize()
-	if len(ciphertext)-size <= 0 {
-		return nil, errors.New("ciphertext is empty")
-	}
-
-	nonce := ciphertext[:size]
-	ciphertext = ciphertext[size:]
-
-	plainText, err := gcm.Open(nil, nonce, ciphertext, nil)
-	if err != nil {
-		return nil, err
-	}
-
-	return plainText, nil
-}