Remove .NET Core 2.1 support, bump up to 1.4.368

[mregen]

• Remove all references to netcoreapp2.1
• Bump up version.json to 1.4.368-preview

[codecov]

Codecov Report

Merging #1645 (21f35e8) into master (7ae5c11) will increase coverage by 0.00%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1645   +/-   ##
=======================================
  Coverage   53.66%   53.67%           
=======================================
  Files         319      319           
  Lines       57740    57740           
=======================================
+ Hits        30988    30991    +3     
+ Misses      26752    26749    -3     

Impacted Files	Coverage Δ
Libraries/Opc.Ua.Client/Session.cs	62.02% <0.00%> (+0.05%)	⬆️
...raries/Opc.Ua.Server/Subscription/MonitoredItem.cs	44.32% <0.00%> (+0.30%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 7ae5c11...21f35e8. Read the comment docs.