Change DeprecatedAlgorithmCheckerTest.py example re #7072

I change it to use DiffractionFocussing which I do not intend to remove for release 3.0, because it is used by a system test.

Signed-off-by: Karl Palmen <karl.palmen@stfc.ac.uk>
Merge branch 'feature/7072_remove_deprecated_algorithms' into develop