vector_sum_of_squares: Only rescale for floating-point numbers

[mhoemmen]

It turns out std::linalg has a couple small wording issues.

#308

The one that practically matters for implementations is LWG 4302.

https://cplusplus.github.io/LWG/lwg-active.html#4302

The reference implementation of vector_sum_of_squares must be fixed.

1. The default algorithm should not attempt rescaling. (Don't try to rescale integers.)
2. Constrain the rescaling algorithm to floating-point numbers.

If LWG 4302 is accepted, that would also make it permissible to use the most recent (post-2021) xLASSQ algorithm, as the scaling factor would no longer be required to be the max absolute value.