Trigger error for the deprecated methods in LegacyExecutionContext.

 Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | yes
| Tests pass?   | yes
| Fixed tickets | #12688
| License       | MIT
| Doc PR        | none
deprecation messages should use fully qualified class names (``LegacyExecutionContext`` is not a class name). You can rely on ``__CLASS__`` to make it easier