v3.0.0

• #19 - TFM updates (@Numpsy)
• #20 - use Environment.ProcessId where available (@Numpsy)
• #22 - enable nullability annotations (@Numpsy)
• #23 - dependency updates (@Numpsy)
• #24 - update xUnit (@Numpsy)
• #25 - updates for Serilog 4 (@Numpsy)