Deprecation warning in DefaultPluginManager::__construct() is too unspecific

Problem/Motivation

In #3265945: Deprecate plugins using annotations and plugin types not supporting attributes, a deprecation warning was added for plugins that don’t specify a plugin attribute.
However, the deprecation message uses __CLASS__ instead of static::class, so the message is always the same:

Not supporting attribute discovery in Drupal\Core\Plugin\DefaultPluginManager is deprecated in drupal:11.2.0 …

This makes it hard to find out which plugin types are actually affected.

Steps to reproduce

Have any plugin type installed that doesn’t support attribute discovery yet.

Proposed resolution

Change __CLASS__ to static::class to include the actually affected plugin manager class in the deprecation message.

Since the current code uses __CLASS__ instead of including the class name verbatim, I think this was the intended behavior, but the difference between __CLASS__ and static::class was not properly understood and the behavior was not properly tested.

Remaining tasks

• Decide whether we want to make this change.
• Implement an MR.
• Review and merge.

User interface changes

None.

Introduced terminology

None.

API changes

None.

Data model changes

None.

Release notes snippet