Config: change default SP version from 2.3 to 2.4

Change the default SP version pre-selected on the registration screen
from 2.3 to 2.4/2.5.  This avoids issues where someone accidentally
leaves the SP version at what it was.

Any new deployments should be using 2.4/2.5 anyway.

2.3 is already seriously old (and dangerous), so also change the label
to say "legacy".