Plugin does not validate or check existence of usernames when managing mute list

[AayushChaudhary0001]

While testing the GitHub plugin, I observed the following issues related to the mute list functionality:

• The plugin does not validate whether the provided username is valid before adding it to the mute list.
• It also does not check whether the username already exists in the mute list before attempting to remove it.

[Yash-Chakerverti]

Fixed in latest release