Replace security/forms/set/export processor with a class based one and fix #12021

### What does it do?

Replace the legacy XML export processor for form customization sets with a class based one which in term fixes the broken export functionality reported in #12021

### Why is it needed?

Restore form customization export functionality and remove one more legacy processor

### Related issue(s)/PR(s)

#12021
#11437 @TheBoxer please update the list there (if this is merged) =)