Add Plex Media Server to Brewfile

I used Plex previously, but switched to Jellyfin about a year ago.
https://jellyfin.org/

Some reasons why I switched:

- They were using a cross-site fingerprinter for a while in their
  Plex Media Server web app. I discovered this with the Brave browser.
- It's difficult to find and configure privacy settings, and you get
  no privacy benefit for subscribing to a premium plan.
- Batch-editing playlists is painful. You can't select and move a range
  of items, only one item can be moved at a time.
- Playlist import from Apple macOS Music/iTunes never worked well.
- Selecting music to sync for offline listening is painful. There's no
  "sync only checked songs" option like there is in macOS Music.
  Conversions for offline syncing are slow.
- Music metadata can't be edited as well as in macOS Music.
- Remote access doesn't work well when the media server computer is on
  a VPN. The connection gets routed through the Plex relay servers, and
  quality is poor.

I still prefer Jellyfin, but it's helpful to have Plex available as an
alternative server, in case Jellyfin isn't working properly.

Plex is starting on a partnership with Next.js that will be cool to see.

Plex has also implemented some nice UX improvements lately:
https://www.plex.tv/blog/choose-your-own-adventure-introducing-modern-layout/
https://www.plex.tv/blog/going-off-grid-just-got-great/
https://www.plex.tv/blog/finding-your-true-colors-with-plex/

Brewfile

@@ -80,6 +80,7 @@ cask "zotero" # no arm64_big_sur yet
 cask "backblaze"
 cask "handbrake"
 cask "jellyfin" # no arm64_big_sur yet
+cask "plex-media-server" # no arm64_big_sur yet
 cask "protonvpn"
 # cask "vscodium" # no arm64_big_sur yet: https://github.com/VSCodium/vscodium/issues/597