https://github.com/rails/rails/pull/39461 changed the `no-store`
directive for the `Cache-Control` header to be exclusive, i.e. when
setting `Cache-Control` to `private, no-store`, this is simplified to
just `no-store`. `private` should typically be superfluous there, but
it's not always.

For instance, Fastly "does not currently respect no-store or no-cache
directives" and says that "if you need to prevent caching by both Fastly
and web browsers, we recommend combining the private directive with
max-age=0 or no-store".

https://docs.fastly.com/en/guides/configuring-caching#do-not-cache

Since it's not possible to override this directive reduction behaviour,
the changes in #39461 prevent Fastly users from upgrading Rails.

This changes the behaviour to allow setting a 'private, no-store' header
when private is specified - similar to how 'public' can be specified
when 'no-cache' is, but not as a default.

Fixes https://github.com/rails/rails/issues/40798