Fix JSON-encoding ActiveStorage::Filename

[jdelStrother]

Motivation / Background

ActiveStorage::Filename was missing quotes when encoded, generating invalid json like this -

JSON.generate(foo: ActiveStorage::Filename.new("bar.pdf") # => '{"foo":bar.pdf}'

Detail

Delete to_json and rely on the implementation from ActiveSupport::ToJsonWithActiveSupportEncoder

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]
• Tests are added or updated if you fix a bug or add a feature.
• CHANGELOG files are updated for the changed libraries if there is a behavior change or additional feature. Minor bug fixes and documentation changes should not be included.