Upgrade to use .NET 6 since PowerShell 7.0 is now out out of support

[bergmeister]

PR Summary

Only PowerShell 7.2 and 7.3 are supported at the moment, which are using .NET 6 and 7, therefore we can bump our version of .NET as well.

fixes #1919

PR Checklist

• PR has a meaningful title
  • Use the present tense and imperative mood when describing your changes
• Summarized changes
• Change is not breaking
• Make sure all .cs, .ps1 and .psm1 files have the correct copyright header
• Make sure you've added a new test if existing tests do not effectively test the code changed and/or updated documentation
• This PR is ready to merge and is not Work in Progress.
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[fflaten]

Remember to bump devcontainer-image as well :)

[peterhoeg]

Doesn't this need to change too:

PSScriptAnalyzer/PSCompatibilityCollector/Microsoft.PowerShell.CrossCompatibility/Microsoft.PowerShell.CrossCompatibility.csproj

Line 5 in 9f456d0

<TargetFrameworks>netstandard2.0;net462</TargetFrameworks>

?

[JamesWTruher]

LGTM

[bergmeister]

Doesn't this need to change too:

PSScriptAnalyzer/PSCompatibilityCollector/Microsoft.PowerShell.CrossCompatibility/Microsoft.PowerShell.CrossCompatibility.csproj

Line 5 in 9f456d0

<TargetFrameworks>netstandard2.0;net462</TargetFrameworks>

?

No, net 462 is for windows powershell and .net 6 still implements .net standard 2