message: Add that view.pdf.backgroundColor is deprecated [skip ci]
diff: ### Changed
  - Configurations `chktex.args.active` and `chktex.args.root` are deprecated in favour of `chktex.exec.args`.
  - Configuration `chktex.args.path` is deprecated in favour of `chktex.exec.path`.
  +- Configuration `view.pdf.backgroundColor` is deprecated in favour of `view.pdf.color.light.backgroundColor` and `view.pdf.color.dark.backgroundColor`.
  ## [8.26.0] - 2022-05-05
  