Updating project compatibility

[NavneetThekkumpat]

Description

Making Microsoft.PowerApps.TestEngine project .netstandard compatible.

.netstandard2.0 -> compatible with .net4.8 project
.netstandard2.1 -> compatible with .net6.0 project

And hence can be consumed by PAC Cli (Need to be compatible with both .net 4.8 and 6.0)

Checklist

• [x] I have performed end-to-end test locally.
• [x] My changes generate no new warnings
• [x] I used clear names for everything
• [x] I have performed a self-review of my own code

[github-actions[bot]]

Package	Line Rate	Branch Rate	Complexity	Health
Microsoft.PowerApps.TestEngine	91%	89%	811	✔
Summary	91% (2149 / 2364)	89% (493 / 556)	811	✔

Minimum allowed line rate is 50%

[arpavan]

This imports into PAC CLI and builds successfully. Good job on this!