Add support for Rails 7 (alpha2)

[smudge]

/domain @samandmoore @effron /no-platform

This:

• Adds an Appraisal entry for 7.0.0.alpha2. (I had to specify multiple gems to get the pre-release dependencies to resolve properly.)
• Bumps .ruby-version to 2.7, but keeps linting at 2.6. (CI will still run against all of 2.6-3.0)
• Fixes a deprecation warning about ActiveRecord::Base.default_timezone (switches to ActiveRecord.default_timezone)
• Fixes some YML-autoloading tests by enabling Zeitwerk in the 7.0 test suite. (The :classic autoloader is no longer used/supported in Rails 7)
• Autocorrects some new linter rules pulled from the latest version of the linter gem
• Bumps the version to 0.3.0 and updates the CHANGELOG

[nanda-prbot]

Needs somebody from @samandmoore and @effron to claim domain review

Use the shovel operator to claim, e.g.:

@myname << domain && platform

_{HOW TO: Claim a Review}

[nanda-prbot]

Needs somebody from @samandmoore and @effron to claim domain review

Use the shovel operator to claim, e.g.:

@myname << domain && platform

_{HOW TO: Claim a Review}

[nanda-prbot]

Needs somebody from @samandmoore and @effron to claim domain review

Use the shovel operator to claim, e.g.:

@myname << domain && platform

_{HOW TO: Claim a Review}

[effron]

<<domainLGTM!

[nanda-prbot]

This PR requires additional review because of new changes

Please get another domain review from @effron, or another reviewer with write access if unavailable.

[effron]

domainlgtm

[nanda-prbot]

Approved! :star2: :nut_and_bolt: :pray: