Change phrasing from `Lock issue` to `Lock discussion`

• Lock issue and Unlock this issue should be changed to Lock discussion and Unlock discussion?.
• Same for merge requests.
• The name of the feature and the functionality of the feature is locking discussions. It is not locking the entire object. Only commenting is locked. So it should be named as such. See https://docs.gitlab.com/ee/user/discussions/#lock-discussions.