security - drop net 6 and 7 from CI

[baywet]

both net6 and 7 are EOL and are not receiving security fixes anymore. These versions should not be used in the pipeline anymore.

https://github.com/microsoftgraph/msgraph-sdk-powershell/blob/4ae82396aaac759b89c646e48a439920fb05d641/.azure-pipelines/common-templates/install-tools.yml#L14

[baywet]

re-opening since this was rolled back by #3171

[baywet]

The hard deadline seems to be April. But it's hard to tell for sure. .NET Core Projects | OneBranch

Also using dotnet 6 until then might result in s360 items. .NET 6 Reached End of Support on November 12, 2024 | E+D Threat & Vulnerability Reporting (TVR)

Our security friends also state there will be no exceptions Frequently Asked Questions (FAQ)

[baywet]

We'll need to major bump as we do so since The first version of PS that supports net 8 is 7.4 What's New in PowerShell 7.4 - PowerShell | Microsoft Learn and our requirement has been PS 7.2 until now