[ClassLoader] Add a deprecation note about UniversalClassLoader class
minor #12762 [ClassLoader] Add a deprecation note about UniversalClassLoader class (davidromani)

This PR was squashed before being merged into the 2.7 branch (closes #12762).

Discussion
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[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        |

Commits
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f216c81 [ClassLoader] Add a deprecation note about UniversalClassLoader class