Use of ALIAS target magic requires newish cmake

We need to work out which cmake versions can support BLT's use of ALIAS style targets.

Older versions get tripped up, see this PR for a few examples:

https://github.com/LLNL/conduit/pull/1233

To be clear -- I am not advocating to support ancient versions (3.9), but we should document what is supported and even throw and error to let folks now what is needed if they try an old version.

Is it CMake 3.21+, or?

Start by reviewing the linked conduit PR 1233 and the BLT CMake entry points that use ALIAS-style targets. Determine the oldest supported CMake version, then document that requirement and define the expected error for older versions.

Written by the indexing model from the issue text.