[ci skip] ActiveStorage guides don't recommend using a deprecated config

[santib]

Motivation / Background

This Pull Request has been created because recently a PR was merged suggesting to use a configuration that was deprecated and will be removed in the next minor release of Rails.

Detail

This Pull Request adjusts the ActiveStorage guides to not mention the deprecated configuration.

Additional information

Deprecation of replace_on_assign_to_many: #42596
Removal of replace_on_assign_to_many: c720b7e

Checklist

Before submitting the PR make sure the following are checked:

• 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.

cc @adrianna-chang-shopify

[adrianna-chang-shopify]

Thank you! ❤️

[santib]

Thanks to you and to @siaw23 for the newsletter which brought this to my attention 😄