Compile Reqnroll.Tools.MsBuild.Generation only with netstandard2.0

[obligaron]

🤔 What's changed?

Change Reqnroll.Tools.MsBuild.Generation to only compile to netstandard2.0 and droped compiling to .net462.

⚡️ What's your motivation?

In #373 we got compilation errors because our netstandard2.0 projects reference assemblies with other versions then the one compiled .net462. This also simplifies the logic.

🏷️ What kind of change is this?

• 🏦 Refactoring/debt/DX (improvement to code design, tooling, etc. without changing behaviour)
• 🐛 Bug fix (non-breaking change which fixes a defect)
• 💥 Breaking change (incompatible changes to the API)

♻️ Anything particular you want feedback on?

📋 Checklist:

• Users should know about my change
  • I have added an entry to the "[vNext]" section of the CHANGELOG, linking to this pull request & included my GitHub handle to the release contributors list.

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[gasparnagy]

This would simplify the things a lot and maybe this is the way to go.
The only thing that comes to my mind is when someone uses a generator that needs to be cross compiled (because a dep does not support .NET Standard). But maybe that works too.
We need to do intensive manual testing for this.