Rename server checkForUpdate

[JonnyWong16]

Description

• Move Release to the server module.
• Rename PlexServer.checkForUpdate() to match camelCase and add depreaction warning to PlexServer.check_for_update().

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• This change requires a documentation update

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have added or updated the docstring for new or existing methods