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/channelargs_test.go

@@ -19,7 +19,7 @@ func (s *MmctlUnitTestSuite) TestGetChannelArgs() {
 
 		s.client.
 			EXPECT().
-			GetChannel(context.Background(), notFoundChannel, "").
+			GetChannel(context.TODO(), notFoundChannel, "").
 			Return(nil, &model.Response{StatusCode: http.StatusNotFound}, notFoundErr).
 			Times(1)
 
@@ -34,7 +34,7 @@ func (s *MmctlUnitTestSuite) TestGetChannelArgs() {
 
 		s.client.
 			EXPECT().
-			GetChannel(context.Background(), badRequestChannel, "").
+			GetChannel(context.TODO(), badRequestChannel, "").
 			Return(nil, &model.Response{StatusCode: http.StatusBadRequest}, badRequestErr).
 			Times(1)
 
@@ -49,7 +49,7 @@ func (s *MmctlUnitTestSuite) TestGetChannelArgs() {
 
 		s.client.
 			EXPECT().
-			GetChannel(context.Background(), forbidden, "").
+			GetChannel(context.TODO(), forbidden, "").
 			Return(nil, &model.Response{StatusCode: http.StatusForbidden}, forbiddenErr).
 			Times(1)
 
@@ -64,7 +64,7 @@ func (s *MmctlUnitTestSuite) TestGetChannelArgs() {
 
 		s.client.
 			EXPECT().
-			GetChannel(context.Background(), errChannel, "").
+			GetChannel(context.TODO(), errChannel, "").
 			Return(nil, &model.Response{StatusCode: http.StatusInternalServerError}, internalServerErrorErr).
 			Times(1)
 
@@ -79,7 +79,7 @@ func (s *MmctlUnitTestSuite) TestGetChannelArgs() {
 
 		s.client.
 			EXPECT().
-			GetChannel(context.Background(), successID, "").
+			GetChannel(context.TODO(), successID, "").
 			Return(successChannel, nil, nil).
 			Times(1)
 
@@ -97,12 +97,12 @@ func (s *MmctlUnitTestSuite) TestGetChannelArgs() {
 
 		s.client.
 			EXPECT().
-			GetTeam(context.Background(), teamID, "").
+			GetTeam(context.TODO(), teamID, "").
 			Return(successTeam, nil, nil).
 			Times(1)
 		s.client.
 			EXPECT().
-			GetChannelByNameIncludeDeleted(context.Background(), channelID, teamID, "").
+			GetChannelByNameIncludeDeleted(context.TODO(), channelID, teamID, "").
 			Return(successChannel, nil, nil).
 			Times(1)