Remove Rails 5.1 support

[kennyadsl]

Description
Our code is still compatible with Rails 5.1 but that version is in its EOL phase, so it's not receiving security patches anymore.

This PR removes support for Rails 5.1, and it's a follow-up of #3333. We should merge this PR after we release the next version, probably 2.10.

Checklist:

• I have followed Pull Request guidelines
• I have added a detailed description into each commit message
• [ ] I have updated Guides and README accordingly to this change (if needed)
• [ ] I have added tests to cover this change (if needed)

[jacobherrington]

I think I am OK with dropping support for Rails 5.1 now that it is EOL.

[tvdeyen]

Are we talking about Rails 5.0 or 5.1? Since 5.1 has not reached end of live according to https://guides.rubyonrails.org/maintenance_policy.html

5.1 is still receiving security patches and 5.0 even severe security patches.

Am I missing something?

[kennyadsl]

@tvdeyen I think that document is outdated, according to https://rubyonrails.org/security/

[kennyadsl]

I think this is the last one: https://github.com/rails/rails/blob/6d68bb5f695414b1801c1c3952ef955a5b0b6c6a/guides/source/maintenance_policy.md

[tvdeyen]

I see. Thanks for pointing me to the correct file 👌

Still, I am hesitant with removing this now. We should at least wait another minor version. I don’t want our users to run into situations where they want to update Solidus for a bug fix or new feature and also force them to upgrade Rails.

Mayor dependency changes like that should always be done separate from bug fixes or mayor new features.

Other than that I am fine with removing it.

[kennyadsl]

@tvdeyen what about printing a deprecation warning if Rails 5.1 is used and just remove it from the CI?

[tvdeyen]

This sounds like a good idea.

[kennyadsl]

I've created #3333 and updated this PR, so it's ready to merge after the next release.