RoR: Update assignment section - Mailers

[WongYC-66]

Because

RoR recently updated its official guide to v8.0, and have restructured its contents.

Current assignment is likely only applicable until v7.1

This PR

• update assignment section to match v8.0 guide.

Issue

Closes #XXXXX

Additional Information

Pull Request Requirements

• I have thoroughly read and understand The Odin Project curriculum contributing guide
• The title of this PR follows the location of change: brief description of change format, e.g. Intro to HTML and CSS lesson: Fix link text
• The Because section summarizes the reason for this PR
• The This PR section has a bullet point list describing the changes in this PR
• If this PR addresses an open issue, it is linked in the Issue section
• If any lesson files are included in this PR, they have been previewed with the Markdown preview tool to ensure it is formatted correctly
• If any lesson files are included in this PR, they follow the Layout Style Guide

[rlmoser99]

@WongYC-66 Thank you for taking the time to make this PR.

@TheOdinProject/rails-path Before I approve this PR, do we want to update these links to be using the v8.0 of the Rails Guides? Or do we want until we have made other changes for Rails 8 and therefore update these links to be using v7 of the Rails Guides?

[JoshDevHub]

@rlmoser99 I personally think it's fine to go ahead and accept these changes, but we also may need to audit other links to the guides throughout the Rails course. They seem to have changed a lot of them.

ActionMailer itself didn't change at all with v8 (source). They've just reorganized the information on its page in the Rails Guides, and the changes in this PR are for updating the sections that learners read to account for that reorganization. There shouldn't be any content specific to Rails 8 in the updated guide (at least not for ActionMailer).