Add build support for .NET 7

[mattleibow]

Description of Change

Detect when .NET 7 SDK is in use and also build a net7.0 TFM.

Bugs Fixed

• New build things

API Changes

None.

Behavioral Changes

None.

Required skia PR

None.

PR Checklist

• [ ] Has tests (if omitted, state reason in description)
• [ ] Rebased on top of main at time of PR
• [ ] Merged related skia PRs
• [ ] Changes adhere to coding standard
• [ ] Updated documentation