Adopt new Slice class format and preservation approach

[bernardnormier]

In "new Slice" (https://docs.icerpc.dev/slice1):

• exceptions are always encoded in the sliced format
• classes encoded in Sliced format are always "fully preserved" (there is no attribute to preserve slices)
• exceptions are not preserved - unknown slices are lost during decoding

I propose we do the same in Ice for Ice 3.8 and cleanup Slice files like: https://github.com/zeroc-ice/ice/blob/ce1649d310904937a3e70a0ac79dc0bcb3a7433a/slice/Glacier2/Session.ice#L37

In this file, we mark operations format:slice ostensibly to encode the one exception (CannotCreateSessionException) in the sliced format even though we don't define anywhere a derived exception.

[bentoi]

Do we keep the 1.0 encoding support for 3.8?

[bernardnormier]

Yes, so far we plan to keep the 1.0 encoding in 3.8. It should have no or limited effect on this issue since slice preservation is 1.1-only.