Update Targeting Policy

[caleblloyd]

Proposed change

Update our targeting policy in the README.md. Currently it reads:

Target .NET Standard 2.0, 2.1, and .NET LTS releases (currently .NET 6.0 & .NET 8.0)

Suggested update:

• Target .NET Standard 2.0, 2.1, and the two most recent .NET LTS releases (currently .NET 6.0 & .NET 8.0)

Use case

When a new .NET LTS target is released, drop targeting for the old .NET LTS target that is out of support. For example, when .NET 10 is released, add support for it and drop support for .NET 6.

Contribution

No response