Size type of `vecmem::jagged_device_vector` should be `unsigned int`

[stephenswat]

This is to bring it in sync with the jagged device vector. Having these two classes have different size types is unlikely to cause any real problems, but it is a little bit of sloppy design that I think we should fix.

[krasznaa]

If you insist on making the two consistent, let's make vecmem::jagged_device_vector::size_type be unsigned int as well...

As it happens, vecmem::device_vector::size_type actually used to be std::size_t. I changed it on purpose when I introduced the ability to resize vectors on the device. This is because atomic operations with CUDA/HIP/SYCL are guaranteed to exist for unsigned int. But not necessarily for std::size_t!

So it's because we use atomic operations on this size that its type was made unsigned int...