Dependency on stable (instead of stable9)

@bernd31, I have fixed the issue and created Patch file. please kindly review the Patch file.

Hi,

Thank you very much, I tried that and it seems I was a bit hasty in my assessment.

There are two problems:

First, after using it like this I got an error message (from update.php I think) that 'stable' was installed but is not included in drupal 10.
To get rid of that one would need to uninstall the (unfortunately hidden core-) stable while still in Drupal9 but after installing Classy. So thats not an easy solution.

Second, on closer inspection I noticed that stable9 is not exactly identical to the old stable. I didn't see any differencesin they layout but in that light I guess one could only recommend using 'at own risk'.

So what I finally did was to also install the contributes stable theme.
Probably that was the intended way to go in the first place. I did however not see any advice to do so.
If I didn't just overlook it could you please think of adding that?

(PS: I'm not sure if composer simply knows about that dependency, since I'm not able to use that)