MM-57245: Bump up Go version to 1.21 (#26512)

* MM-57245: Bump up Go version to 1.21

https://mattermost.atlassian.net/browse/MM-57245
```release-note
NONE
```

server/cmd/mmctl/commands/integrity_test.go

@@ -41,7 +41,7 @@ func (s *MmctlUnitTestSuite) TestIntegrityCmd() {
 		}
 		s.client.
 			EXPECT().
-			CheckIntegrity(context.Background()).
+			CheckIntegrity(context.TODO()).
 			Return(mockResults, &model.Response{}, nil).
 			Times(1)
 
@@ -59,7 +59,7 @@ func (s *MmctlUnitTestSuite) TestIntegrityCmd() {
 
 		s.client.
 			EXPECT().
-			CheckIntegrity(context.Background()).
+			CheckIntegrity(context.TODO()).
 			Return(nil, &model.Response{}, errors.New("mock error")).
 			Times(1)
 
@@ -99,7 +99,7 @@ func (s *MmctlUnitTestSuite) TestIntegrityCmd() {
 		}
 		s.client.
 			EXPECT().
-			CheckIntegrity(context.Background()).
+			CheckIntegrity(context.TODO()).
 			Return(mockResults, &model.Response{}, nil).
 			Times(1)
 		var expected error