Guides still import on, fn, etc

Ember 7.1 now includes several built-in helpers/modifiers that were previously imported. The docs should be updated to reflect the new template behavior.

TODO:
- Identify the code blocks (`gjs`/`gts`) where helpers/modifiers are imported and remove the imports
- Update the [section on importing built-ins](https://guides.emberjs.com/release/components/template-tag-format/#toc_importing-built-ins)
- Update the [section on keywords](https://guides.emberjs.com/release/components/template-tag-format/#toc_keywords)
- Possibly add some prose describing that behavior changed as of 7.1
- Update the [section on helper functions](https://guides.emberjs.com/release/components/helper-functions/#toc_built-in-and-common-helpers), particularly the wording on the CTA.

Search the guides for gjs/gts code blocks that import built-in helpers or modifiers, then read the linked sections on importing built-ins, keywords, and built-in and common helpers. Remove obsolete imports and update the referenced prose, including the 7.1 behavior change and CTA wording; done means the examples and guidance match Ember 7.1.

Written by the indexing model from the issue text.