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.

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Reviewer Checklist:

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