Post excerpt: Update read more placeholder text

[carolinan]

Description

For #29168
Update the post excerpts read more placeholder from "Read more..." to Add "read more" link text

How has this been tested?

By manually adding a post excerpt block

Screenshots

Types of changes

Enhancement

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• I've tested my changes with keyboard and screen readers.
• My code has proper inline documentation.
• I've included developer documentation if appropriate.
• I've updated all React Native files affected by any refactorings/renamings in this PR (please manually search all *.native.js files for terms that need renaming or removal).

[kristastevens]

Hello @carolinan, thanks for the ping! I have one small suggestion, to add quotations around "read more" in your CTA:

Add "read more" link text

I think adding the quotation marks might help a little with understanding the action they need to take here.