Change call of FlatBackground to CalculateFlatBackgound re #6977

This was done in PolrefExample.py.

Signed-off-by: Karl Palmen <karl.palmen@stfc.ac.uk>
Update results reference file re #6977

This was necessary because of changes made to CalculateFlatBackground to ensure it did not use a deprecated child algorithm.

Signed-off-by: Karl Palmen <karl.palmen@stfc.ac.uk>
Merge remote-tracking branch 'origin/feature/6977_remove_flatbackgound_alg'