[DEADCODE] 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)
(cherry picked from commit 0de9d18863)
(cherry picked from commit 26abf78374)
(cherry picked from commit 05d3a143c3)
(cherry picked from commit 4b3d38d5e1)
(cherry picked from commit a726e71986)
(cherry picked from commit cb62ae5b98)
(cherry picked from commit 8195ba06d5)
(cherry picked from commit 4570fb591a)
(cherry picked from commit 1f4d33de2b)

modules/util/legacy_test.go

@@ -4,8 +4,6 @@
 package util
 
 import (
-	"crypto/aes"
-	"crypto/rand"
 	"fmt"
 	"os"
 	"testing"
@@ -37,21 +35,3 @@ func TestCopyFile(t *testing.T) {
 	assert.NoError(t, err)
 	assert.Equal(t, testContent, dstContent)
 }
-
-func TestAESGCM(t *testing.T) {
-	t.Parallel()
-
-	key := make([]byte, aes.BlockSize)
-	_, err := rand.Read(key)
-	assert.NoError(t, err)
-
-	plaintext := []byte("this will be encrypted")
-
-	ciphertext, err := AESGCMEncrypt(key, plaintext)
-	assert.NoError(t, err)
-
-	decrypted, err := AESGCMDecrypt(key, ciphertext)
-	assert.NoError(t, err)
-
-	assert.Equal(t, plaintext, decrypted)
-}