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

• [x] PR has a meaningful title
  • Use the present tense and imperative mood when describing your changes
• [x] Summarized changes
• [x] Change is not breaking
• [x] 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
• [x] This PR is ready to merge and is not Work in Progress.
  • If the PR is work in progress, please add the prefix WIP: to the beginning of the title and remove the prefix when the PR is ready.

[fflaten]

Remember to bump devcontainer-image as well :)

[peterhoeg]

Doesn't this need to change too: https://github.com/PowerShell/PSScriptAnalyzer/blob/9f456d05b95421090fa63e1bb7378231291b36b6/PSCompatibilityCollector/Microsoft.PowerShell.CrossCompatibility/Microsoft.PowerShell.CrossCompatibility.csproj#L5

?

[bergmeister]

Doesn't this need to change too:

https://github.com/PowerShell/PSScriptAnalyzer/blob/9f456d05b95421090fa63e1bb7378231291b36b6/PSCompatibilityCollector/Microsoft.PowerShell.CrossCompatibility/Microsoft.PowerShell.CrossCompatibility.csproj#L5

?

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