[Admin UI extensions 2025-07]: Consistent casing for admin UI extensions term

[mcvinci]

This PR:

• Standardizes the casing of "admin UI extensions" / "admin UI extension" to use lowercase "admin" when the term appears mid-sentence across content .ts files
• Keeps capitalized "Admin UI extensions" only when it's at the start of a sentence, in a heading, or in a title attribute
• Partially fixes https://github.com/Shopify/shopify-dev/issues/68908

[github-actions]

We detected some changes in packages/*/package.json or packages/*/src, and there are no updates in the .changeset directory. If the changes are user-facing and should cause a version bump, run yarn changeset to track your changes and include them in the next release CHANGELOG. If you are making simple updates to repo configuration, examples, or documentation, you do not need to add a changeset.