Fix deprecation warning messages for ActiveSupport::TestFixtures

[adrianna-chang-shopify]

Motivation / Background

While reviewing @zzak 's documentation improvements around fixture paths, I noticed a few minor inconsistencies with the deprecation messages.

Detail

Minor fixes to the deprecation messages around TestFixtures.fixture_path, TestFixtures.fixture_path=, and TestFixtures#fixture_path to clarify which method is deprecated and which should be used instead.

Ensure we refer to the class method vs instance method appropriately, and refer to the setter method in the deprecation method for TestFixtures.fixture_path=.

cc @andrewn617

Checklist

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• This Pull Request is related to one change. Changes that are unrelated should be opened in separate PRs.
• Commit message has a detailed description of what changed and why. If this PR fixes a related issue include it in the commit message. Ex: [Fix #issue-number]