fix: make FFmpegOpusAudio codec behave as docs suggest

[felix920506]

Summary

make FFmpegOpusAudio "codec" parameter behave as what the inline docs suggest

Original behavior:
passing in "libopus" or "opus" will result in "copy" being passed into ffmpeg
passing in anything else (including "copy") will result in "libopus" being passed into ffmpeg

New behavior:
passing in "copy" will result in "copy" being passed into ffmpeg
passing in anything else (including "libopus", "opus") will result in "libopus" being passed into ffmpeg

Information

• 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, ...).

Checklist

• I have searched the open pull requests for duplicates.
• 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.
• I have updated the changelog to include these changes.

[plun1331]

should we maybe change the docs instead because like, breaking changes and stuff

[Lulalaby]

It's more an internal change. So I wouldn't consider it breaking

[felix920506]

This is my first contribution here so I don't really know if this would count as breaking or not. I checked the box only because it technically changes the behavior of this function.

[felix920506]

I don't think this is going to be much an issue since the only scenario it would break functionality on existing bots would be someone passing in "copy" for a source that isn't opus, which is something that was never supposed to work according to the docs.
The fix this will require under that scenario would be a few lines at most (Likely single line changes for most cases.)

[felix920506]

I see this tagged with "changelog needed", is there anything that I need to do? If so are there instructions?

[JustaSqu1d]

I see this tagged with "changelog needed", is there anything that I need to do? If so are there instructions?

Here's an example of a changelog entry being added: 7fb1a4b

[felix920506]

I see this tagged with "changelog needed", is there anything that I need to do? If so are there instructions?

Here's an example of a changelog entry being added: 7fb1a4b

would this be "Fixed" or "Changed" ?

[Lulalaby]

Fixed

[felix920506]

I have added an entry to the changelog.