fix(brew): Rename postflight to postflight_steps for puremac cask

[quirico]

postflight is deprecated. Changed to postflight_steps to suppress brew warning while updating:

Warning: Calling `postflight` is deprecated! Use `postflight_steps` instead.
Please report this issue to the momenbasel/homebrew-tap tap (not Homebrew/* repositories), or even better, submit a PR to fix it:
  /opt/homebrew/Library/Taps/momenbasel/homebrew-tap/Casks/puremac.rb:16

What does this PR do?

Replace deprecated postflight with postflight_steps in homebrew script

Type of change

• Bug fix
• New feature
• Performance improvement
• UI/UX enhancement
• Documentation
• Other (describe)

Testing

• Tested on macOS Ventura (13.x)
• Tested on macOS Sonoma (14.x)
• Tested on macOS Sequoia (15.x)

Screenshots