Make ProjectCoreCompatibility class final and internal

I guess I'm +1 to this change. However, this bit from the summary seems a little unfortunate:

If some needs the compatiblity message they should call update_calculate_project_data().
If they want a different message they can implement hook_update_status_alter() and unset core's message and make their own. Our message should not be extendable besides translation.

I thought we hated the procedural "API" (such as it is) for Update module. Seems like a wonky thing to recommend people do if they need this info. I know that's a whole bigger / different problem.

Certainly #3113992: The 'Update' page has no idea that some updates are incompatible can do what it needs without touching this class. So that's a good argument that this class really doesn't need to be called directly by anyone outside of update_calculate_project_data().

So, okay, I guess I convinced myself. Let's do this.

Upon closer review:

1. +++ b/core/modules/update/src/ProjectCoreCompatibility.php
   @@ -8,8 +8,12 @@
     * Utility class to set core compatibility messages for module updates.
   

   This line of context is changing upstream, so this patch probably won't apply cleanly after #3102724: Improve usability of core compatibility ranges on available updates report lands. Not much we can do about it now, but I'd be in favor of postponing this on making sure that lands first so we don't derail anything...

2. +++ b/core/modules/update/src/ProjectCoreCompatibility.php
   @@ -8,8 +8,12 @@
   + * @internal
   + *   This class implements internal logic for the Update module. It should not
   + *   be called directly.
   

   This comment doesn't add anything beyond what @internal already says. I'm not sure what to do about that. I believe our standards (not sure they're documented) are to have a comment with @internal declarations. But saying, in effect: "this is internal and is internal and should not be called externally" seems pointless.

How about something like this for the comment:

 * @internal
 *   This class implements logic used by update_calculate_project_status().
 *   It should not be called directly.
 *
 * @see update_calculate_project_status()

(approx).

Thanks,
-Derek

Looks great

Hate to do this for ridiculous nits, but I suspect the core committers will notice, so we might as well fix it, first:

+++ b/core/modules/update/src/ProjectCoreCompatibility.php
@@ -8,8 +8,12 @@
+ * @internal
+ *   This class implements logic used by update_calculate_project_status(). It
+ *    should not be called directly.

Improper indenting (extra space) on the 2nd line of the comment.

Otherwise, all +1 here.

Thanks!
-Derek

Fixed #11.
My preference would be for #3102724: Improve usability of core compatibility ranges on available updates report to land in 8.8.x first, then the 8.9.x patch here should cleanly apply.
Otherwise, here's an 8.8.x-specific patch if needed.

I agree it makes sense to be more limited in terms of our API surface with stuff we do not intend people to use. Good call.