VS: Fix '-T version=14.28' under VS 16.9

CMake accepts the toolset version that is default in the current VS
version by matching the name later VS versions will use for the SxS
props files.  It predicts the future name based on the first two
components of the current VS version's default toolset.  However, this
heuristic breaks naming the VS 16.8 toolset version 14.28 under VS 16.9
because the latter's default toolset version is 14.28.29910, which did
not increment the second version component (unprecedented in VS).

Fix this by always using the requested version's SxS props file when it
exists, even if it matches the first two components of the current VS
version's default toolset.  Also add a special case for the name VS
16.10 will use for VS 16.9's default toolset, so that it can be used
with VS 16.9 too.

Fixes: #21922