Cask: Calling `postflight` is deprecated

[pmer]

The postflight command seems to be now deprecated by Homebrew:

$ brew install --cask lstk
Warning: Calling `postflight` is deprecated! Use `postflight_steps` instead.
Please report this issue to the localstack/homebrew-tap tap (not Homebrew/* repositories), or even better, submit a PR to fix it:
  /opt/homebrew/Library/Taps/localstack/homebrew-tap/Casks/lstk.rb:39

==> Would install 1 cask:
localstack/tap/lstk
==> Fetching downloads for: localstack/tap/lstk
✔︎ Cask lstk (1.0.0)                                                     Downloaded    7.8MB/  7.8MB
==> Installing Cask lstk
Warning: Calling `postflight` is deprecated! Use `postflight_steps` instead.
Please report this issue to the localstack/homebrew-tap tap (not Homebrew/* repositories), or even better, submit a PR to fix it:
  /opt/homebrew/Library/Taps/localstack/homebrew-tap/Casks/lstk.rb:39

==> Linking Binary 'lstk' to '/opt/homebrew/bin/lstk'
==> Linking Bash Completion 'lstk.bash' to '/opt/homebrew/etc/bash_completion.d/lstk'
==> Linking Fish Completion 'lstk.fish' to '/opt/homebrew/share/fish/vendor_completions.d/lstk.fish'
==> Linking Zsh Completion 'lstk.zsh' to '/opt/homebrew/share/zsh/site-functions/_lstk'
🍺  lstk was successfully installed!
Warning: Calling `postflight` is deprecated! Use `postflight_steps` instead.
Please report this issue to the localstack/homebrew-tap tap (not Homebrew/* repositories), or even better, submit a PR to fix it:
  /opt/homebrew/Library/Taps/localstack/homebrew-tap/Casks/lstk.rb:39

[joe4dev]

Thank you for reporting @pmer

Reproducer

I can confirm the issue is now fixed (no more deprecation warning) after merging #8