Upgrade projects to .Net 6

[tusmester]

We still have projects on netstandard, netcore, etc. Try to consolidate these to .Net 6 (which is the current LTS version).

• [ ] update build agents
• [ ] sensenet github repository

Exceptions (compatibility reasons)

• .net client library: stay on netstandard if possible

What about packages not in this repository?

• tools
• security
• Lucene search
• preview
• task management