Drop the use of deprecated ruby-growl gem

[utkarsh2102]

Hi,

uniform_notifier/uniform_notifier.gemspec

Line 20 in 5827f1e

s.add_development_dependency 'ruby-growl', ['= 4.0']

The upstream of ruby-growl gem is dead and there are test failures (since it doesn't work with Ruby2.7). Can you perhaps drop their usage and find an alternative?

[mishina2228]

ruby-growl is fairly outdated, and Growl itself has already been retired.
https://github.com/growl/growl/#retirement

Growl is a retired project, we couldn't think of another thing to change which would be substantial enough to bring out a new updated release.

I assume that not many people are using Growl now, so I think we can stop supporting Growl.

[flyerhzm]

@mishina2228 good to know, I'll drop Growl support soon.

[flyerhzm]

Growl is already removed