Update deprecated/incorrect endpoints

[baronkobama]

Summary

This PR does two things:

• Removes the deprecated endpoint /guilds/{guild_id}/members/@me/nick and replaces it with /guilds/{guild_id}/members/@me in HTTPClient.change_my_nickname.
• Changes the incorrect POST HTTP method to the correct PUT HTTP method in HTTPClient.join_thread.

References:

• Modify Current User Nick (Deprecated) -> Modify Current Member
• Join Thread

Checklist

• If code changes were made then they have been tested.
  • I have updated the documentation to reflect the changes.
• If type: ignore comments were used, a comment is also left explaining why
• This PR fixes an issue.
• This PR adds something new (e.g. new method or parameters).
• This PR is a breaking change (e.g. methods or parameters removed/renamed)
• This PR is not a code change (e.g. documentation, README, typehinting, examples, ...)

Note

• For some reason, it appears that the POST HTTP method does function for HTTPClient.join_thread, as well as PUT. Gonna go ahead and change to the currently documented method (PUT) anyways. 🤷