codeowners, maintainers: update my status on a few areas

This patch updates my status on areas where unfortunately I no longer
have time or interest to contribute:

- Removed my collaborator status from STM32, display, sensors, kscan,
  PWM and hal_ti
- Downgraded my status from maintainer to collaborator for the GD32
  platform

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>