Implement Bad Gateway handling & code cleanup

Yes, it's that time of year again!

No not christmas, a new PR for the opensubtitles plugin

Changes:

Added handling for 502 bad gateway (https://forum.opensubtitles.org/viewtopic.php?f=8&p=46460#p46460)
Removed some properties that weren't being used/were removed by opensubtitles
DownloadSubtitleAsync no longer uses RequestHandler which makes stuff much more cleaner
Added max number of retries for 429 responses
Removed OpenSubtitlesHandler.csproj & tiny readme change

jellyfin / jellyfin-plugin-opensubtitles