CI against Ruby 3.0

[tbotaq]

Hi.

I've tried some fix to make the CI status green against Ruby 3.

Removing :rbx specifications from .gemfile

It seems that the version specification of rubysl doesn't allow Ruby 3.0 (See: the failing CI's log ).
I've tried to find some web source about 3.0 support of rubysl but I could not.

By the way, it also seems that our CI doesn't list rbx as its build target.

So I found it not necessary to get the problem fixed, and removed platforms :rbx specifications from all the .gemfile.
Is it OK ? I'm not certain if it's ok for this repository.

---

I'm glad if you see the commit comments to figure out what I did.
Thank you.

ref: #225

[tbotaq]

rbx has been dropped from our build target since this commit .

Can we also remove rbx from gemfiles like this ?
（Even bundle install fails with Ruby 3 like this）

[tbotaq]

Excluding these Rails 5.x and Ruby 3 combinations from the build target.

Because Rails 5 doesn't support Ruby 3.
See: rails/rails#40938

[tbotaq]

• Rebased on the current master
• Left some comments on the source diff of this PR.（Sorry for too-long description !）

[kbrock]

delete any old gem files and then lets merge/call it a day

[tbotaq]

Thank you for the comment !

Let me ask,

delete any old gem files

Q: Which gemfiles should we regard as old ?

README says,

To make users' lives easier, please maintain support for:
Ruby 2.4
ActiveRecord/ActiveSupport from 5.0 through edge

So I thought we shouldn't remove any of these existing gemfiles, whose versions are 5.0 through edge .

rails_5.0.gemfile
rails_5.1.gemfile
rails_5.2.gemfile
rails_6.0.gemfile

[tbotaq]

• Looks like no gemfile is old
• CI green

So, could anybody merge this PR ?

[tbotaq]

Thank you for merging.