update library projects to target .NET8.0 on the version/4.5 branch

[justadreamer]

Background

This is partially already done for Test projects (see previous commits changing net6.0 target to net8.0), but libraries projects in this repo need to be converted too to use .NET 8.0.

Branching

This needs to be done for the version 4.5 branch so - please create a version/4.5 branch from the current state of the main branch. I will ask @pi-alexander-popel to setup nightly-pipeline build system for pipeline-dotnet/version/4.5.

Objective

subj please and please create a pull request to version/4.5 branch, not to the main one.