encoding: Fix low bitrate check

[Penwy]

Description

This fixes the "Low Stream Bitrate" check to work with the new encoder names, and reworks it to work with all of obs' default encoders, and be less disruptible.

Motivation and Context

The "Low Stream Bitrate" check is currently nonfunctional, due to the encoders rename in OBS 28.

How Has This Been Tested?

This was tested by manually feeding the lines to the function and checking it gets the right values.
It was tested with

• x264
• "new" nvenc and ffmpeg nvenc
• qsv11
• texture amf and fallback amf

Types of changes

Checklist:

• My code has been run through clang-format.
• I have read the contributing document.
• My code is not on the master branch.
• The code has been tested.
• All commit messages are properly formatted and commits squashed where appropriate.
• I have included updates to all appropriate documentation.