Remove .NET 6

[grochocki]

Description:

Further cleanup of framework targeting

Checklist:

• I have commented my code, particularly in hard-to-understand areas
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• I have tested my changes by running the extension in VS2017
• I have tested my changes by running the extension in VS2019
• I have tested my changes by running the extension in VS2022
• If changes to the documentation are needed, I have noted this in the description above