Merge topic 'update-to-v5.9.1'

aad4b6f1 Update version number to 5.9.1

Acked-by: Kitware Robot <kwrobot@kitware.com>
Acked-by: Cory Quammen <cory.quammen@kitware.com>
Merge-request: !4942

version.txt

-5.9.1-RC2
+5.9.1