mgr/dashboard: NFS non-existent files cleanup

mgr/dashboard: NFS non-existent files cleanup

After https://github.com/ceph/ceph/pull/42526 and https://github.com/ceph/ceph/pull/43725 merges,
the following files do not exist but there were still references to them:
- src/pybind/mgr/dashboard/services/ganesha.py
- qa/tasks/mgr/dashboard/test_ganesha.py

The following files were renamed but there were still references to old names:
- src/pybind/mgr/dashboard/controllers/nfsganesha.py:  nfsganesha.py --> nfs.py
- src/pybind/mgr/dashboard/tests/test_ganesha.py:  test_ganesha.py --> test_nfs.py

Other changes in qa/suites/rados/dashboard/tasks/dashboard.yaml:
- Add missing task: tasks.mgr.dashboard.test_api
- Sort dashboard tasks alphabetically.

Fixes: https://tracker.ceph.com/issues/53123
Signed-off-by: Alfonso Martínez <almartin@redhat.com>