Warning on build: rubyforge_project= is deprecated and ignored.

[ssg47]

When building with a default installation of Ruby 2.7.5-1-x64-msvcrt with MSYS2 a warning is generated:

PS D:\dev\repos\slacker> gem build slacker.gemspec
WARNING:  rubyforge_project= is deprecated and ignored. Please remove this from your gemspec to ensure that your gem continues to build in the future.
WARNING:  See https://guides.rubygems.org/specification-reference/ for help
  Successfully built RubyGem
  Name: slacker
  Version: 1.0.25
  File: slacker-1.0.25.gem