[ClassLoader] Add a deprecation note about UniversalClassLoader class

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | yes
| Tests pass?   | yes
| Fixed tickets | #12600
| License       | MIT
| Doc PR        | 
This is actually not correct. The class is only deprecated in 2.7. It will be removed in 3.0.