Rails 6.1

[lgebhardt]

Adds support for rails 6.1

Brings in the rails 6.1 work from #1346, fixes a few JoinManager tests to account for new aliasing behavior, and updates the testing matrix. I'd like to rewrite the JoinManger tests, but not for this PR since it will be a lot of time I's rather spend on new features.

All Submissions:

• I've checked to ensure there aren't other open Pull Requests for the same update/change.
• I've submitted a ticket for my issue if one did not already exist.
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• I've added/updated tests for this change.

New Feature Submissions:

• I've submitted an issue that describes this feature, and received the go ahead from the maintainers.
• My submission includes new tests.
• My submission maintains compliance with JSON:API.

Bug fixes and Changes to Core Features:

• I've included an explanation of what the changes do and why I'd like you to include them.
• I've provided test(s) that fails without the change.

Test Plan:

Reviewer Checklist:

• Maintains compliance with JSON:API
• Adequate test coverage exists to prevent regressions

[nadnoslen]

Thanks @lgebhardt! 🚀