Add ruby 3.0.0 support

[durcak]

What language does this apply to?
ruby

Describe the problem you are trying to solve.
Ruby 3.0.0 was released, which is not supported:

activeadmin-searchable_select-1.2.1 requires ruby version ~> 2.1, which is incompatible with the current version, ruby 3.0.0p0

Describe the solution you'd like
The activeadmin-searchable_select gem is installable under ruby 3.0.0.

[tf]

Opened a PR, but we still need to update the Appraisal setup to use Rails and Active Admin versions that are compatible with Ruby 3. Which versions are you using in the app where you want to use this gem?

[durcak]

Rails 6.1.0 and activeadmin 2.9.0, thanks.

[tf]

Added Rails 6.x to the CI matrix. Appears to fail because of a Ruby 3 compat issue in Formtastic. Are you seeing that issue in your app as well?

[durcak]

Yes!

[tf]

Ok, then we will probably have to wait until Active Admin/Formastic are Ruby 3 compatible before we can move forward here.

[durcak]

Formtastic 4.0.0 has been released and its Ruby 3.0 compatible, can we move forward here?

[tf]

Good to hear. #26 passed now. Merged and released as 1.3.