frontend, plugins: Set default bitrate to 6000 kbps

The default of 2500 kbps was chosen 10 years ago and times have changed.
Logs and forums posts show that many users of OBS for recording don't
change their bitrate and end up with a 2.5 mbps recording which looks
terrible.

Now that service bitrate enforcement exists, this will be automatically
capped to the maximum bitrate for streaming services, so the only time
this should result in a problem is if the user's upload speed is the
limiting factor, hopefully rarer these days with increasing internet
speeds.
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Gitea Version: 1.28.0+dev-477-g8b6ad49a5f