Add % unit to Schneider TRV "display brightness"

[TheJulianJES]

Proposed change

This adds the "percentage" unit to the Schneider thermostat "Display brightness" entities.

Additional information

This is now possible due to the merge of zigpy/zha#352
Follow-up to original PR #3575

Checklist

• The changes are tested and work correctly
• pre-commit checks pass / the code has been formatted using Black
• Tests have been added to verify that the new code works

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.90%. Comparing base (adb16ad) to head (02dfd08).
Report is 16 commits behind head on dev.

Additional details and impacted files

@@           Coverage Diff           @@
##              dev    #3739   +/-   ##
=======================================
  Coverage   89.90%   89.90%           
=======================================
  Files         320      320           
  Lines       10416    10416           
=======================================
  Hits         9365     9365           
  Misses       1051     1051           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.