PSReadLine projects targets .NET 6 which is out of support

[Frulfump]

Prerequisites

• Write a descriptive title.

Description of the new feature/enhancement

The TargetFrameworks is <TargetFrameworks>net472;net6.0</TargetFrameworks>
https://github.com/PowerShell/PSReadLine/blob/master/test/PSReadLine.Tests.csproj
https://github.com/PowerShell/PSReadLine/blob/master/Polyfill/Polyfill.csproj
https://github.com/PowerShell/PSReadLine/blob/master/MockPSConsole/MockPSConsole.csproj

Should be updated to net8.0 instead

Proposed technical implementation details (optional)

Change TFM and fix any issues caused by that
<TargetFrameworks>net472;net8.0</TargetFrameworks>
Might make sense (or be required) to update some packages at the same time.
Update buildscript/readme