Make entity route providers public instead of internal

Problem/Motivation

Follow-up to #2578955: Implement auto route generation but DON'T use it for all core entities. In that issue,

@Crell suggests in #37

Making this class @internal makes little sense to me, as the point of it is that custom entity authors SHOULD use it rather than avoid its existence as an implementation detail. That said, catch is correct that it's much easier for us to @internal->normal than vice versa, so if the plan is to do that during RC in case we need to tweak something else while fleshing out the other entities

@catch suggests in #39

Yes the idea is to un-@internal it before the last release candidate - this for me is a very good use for @internal.

At least 3 minor versions is probably enough time for the API to have stabilized. It looks like some contrib developers are extending theses classes despite the @internal (as recommended above).

Proposed resolution

Remove @internal from DefaultHtmlRouteProvider and AdminHtmlRouteProvider classes.
Update / add change record?

Remaining tasks

Write a patch.
Review patch.

User interface changes

No

API changes

Yes, removes @internal that was supposed to be removed in 8.0.0.

Data model changes

No.