Rename "cross-targeting" to "multi-targeting"

[mhutch]

Per internal discussion, rename the "cross-targeting" feature to "multi-targeting".

Rationale

Cross-targeting is easily confused with cross-compilation which has a well-defined and quite different meaning.

Multi-targeting describes the feature very well - the ability for a project to compile for multiple target frameworks. Although it has an existing meaning in MSBuild (the ability for a project to target different frameworks), using it this way this is an expansion of scope, not a redefinition.