[XRay] Unsupport version<2 sled entry

For many features we expect clang and compiler-rt to have a version lock
relation, yet for XRaySledEntry we have kept version<2 compatibility for more
than 2 years (I migrated away the last user mips in 2020-09 (D87977)).
I think it's fair to call an end to version<2 now. This should discourage more
work on version<2 (e.g. D140725).

Reviewed By: ianlevesque

Differential Revision: https://reviews.llvm.org/D140739