fix: update deprecated homebrew syntax to use postflight_steps

[jackson-hughes]

Important

This PR depends on an upcoming version of goreleaser, likely v2.19.0

Closes: #146
Depends on: goreleaser/goreleaser#6873

I changed the goreleaser config to use install_steps which should remove the warning being printed:

Warning: Calling `postflight` is deprecated! Use `postflight_steps` instead.
Please report this issue to the postfinance/homebrew-tap tap (not Homebrew/* repositories), or even better, submit a PR to fix it:
  /opt/homebrew/Library/Taps/postfinance/homebrew-tap/Casks/topf.rb:36

[clementnuss]

thanks, that looks appropriate! can you let us know once v2.19 ships?