Split the JUnit 5 `OutputCapture` class into separate `OutputExtension`
and `CapturedOutput` classes. The JUnit 5 callback methods are now
contained only in the `OutputExtension` class so no longer pollute the
public API that users will interact with.
The `CapturedOutput` class has also been updated to capture System.err
and System.out separately to allow distinct assertions if required.
Closes gh-17029
d11d5ceb29