Remove deprecated color_mode flag, adjust brightness description for MQTT json light

[jbouwh]

Proposed change

Remove deprecated color_mode flag, adjust brightness description for MQTT json light

Type of change

• Spelling, grammar or other readability improvements (current branch).
• Adjusted missing or incorrect information in the current documentation (current branch).
• Added documentation for a new integration I'm adding to Home Assistant (next branch).
  • I've opened up a PR to add logos and icons in Brands repository.
• Added documentation for a new feature I'm adding to Home Assistant (next branch).
• Removed stale or deprecated documentation.

Additional information

• Link to parent pull request in the codebase: Deprecate old config options for MQTT json light core#111676
• Link to parent pull request in the Brands repository:
• This PR fixes or closes issue: fixes #

Checklist

• This PR uses the correct branch, based on one of the following:
  • I made a change to the existing documentation and used the current branch.
  • I made a change that is related to an upcoming version of Home Assistant and used the next branch.
• The documentation follows the Home Assistant documentation standards.

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[home-assistant]

Hey there @emontnemery, mind taking a look at this feedback as it has been labeled with an integration (mqtt) you are listed as a code owner for? Thanks!

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[c0ffeeca7]

Thank you, @jbouwh 👍