`readonly` has been marked private since 2016

[kategengler]

ember.js/packages/@ember/-internals/glimmer/lib/helpers/readonly.ts

Line 118 in 452b901

@private

The readonly template helper has been marked private since 2016. However, it does not seem like we deprecated it and it continues to work.

I see two options:

1. Restore the docs to public
2. Add a deprecation for intimate API until the next LTS

I do not believe using readonly is still necessary in any but very old code using ember component.

[NullVoxPopuli]

big fan of deprecating