Move QuicIntervalSet:Set allocator template argument to internal platform impl.

Also rename QuicOrderedSet to QuicSmallOrderedSet to reflect that this
container is optimized for small sets.

Before this change Set is some_internal_container<absl::InlinedVector>
(preferred) internally and absl::btree_set<std::vector> in Chromium.

After this change Set is the same internally but absl::btree_set (without the
extra allocator argument) in Chromium.

Chromium's QuicOrderedSetImpl should later be changed to base::flat_set, which
aligns better with the performance properties of the internal implementation
(and the performance requirements of QuicIntervalSet) than absl::btree_set.

This change unblocks changing Chromium's QuicInlinedVectorImpl from std::vector
to absl::InlinedVector, which is currently not possible because
absl::btree_set<absl::InlinedVector> does not compile.
PiperOrigin-RevId: 379947057