Support huge cluster containers for VP / UT / FT

So far, we supported events which had up to 2^16 = 65k clusters.

In the processing of 2024 data, some events were encountered with many, many more clusters. While these are pathological events that (I think) we want to reject with a GEC, there was actually not a very strict reason to have the 65k cluster limit. This shifts that limit to the maximum possible size of an std::vector, instead.

Needed to fix Rec#566

Kernel/LHCbKernel/include/Kernel/MultiIndexedContainer.h

@@ -122,7 +122,7 @@ namespace LHCb::Container {
   template <class Hit, size_t... sizes>
   class MultiIndexedContainer {
   public:
-    using offset_t        = unsigned short;
+    using offset_t        = size_t;
     using const_reference = const Hit&;
     using value_type      = Hit;
     using allocator_type  = LHCb::Allocators::EventLocal<value_type>;