Add support for Ruby 3

[rzane]

Summary

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request... were there any bugs you had fixed? If so, mention them. If
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TestTrack::AssignmentEventJob expects keyword arguments, but TestTrack::UnsyncedAssignmentsNotifier is passing a hash. This PR splats the hash to make this gem compatible with Ruby 3.

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Thanks for contributing to TestTrack!

/domain @Betterment/test_track_core
/platform @Betterment/test_track_core

/no-platform

[jesseproudman]

domain lgtm

[jesseproudman]

Do we need to bump the version?

https://github.com/Betterment/test_track_rails_client/blob/master/lib/test_track_rails_client/version.rb

Dismissed because the approval was invalidated by another commit

[jesseproudman]

domain lgtm

[smudge]

Worth adding 3.1 while we're here?

[rzane]

I was going to, but Rubocop failed with this issue. We need to bump Rubocop, which causes new cops to fail. I'd prefer to do that in a separate PR.

Dismissed because the approval was invalidated by another commit

[smudge]

domain LGTM && platform LGTM!

[smudge]

domain LGTM