Move to .net 6

[david-driscoll]

• Updated build and test dependencies
• Move to incremental source generators
• Dropped netcoreapp2.1 targets

[codecov]

Codecov Report

Merging #757 (1bffa0c) into master (9ddacc0) will increase coverage by 0.25%.
The diff coverage is 83.66%.

@@            Coverage Diff             @@
##           master     #757      +/-   ##
==========================================
+ Coverage   72.96%   73.21%   +0.25%     
==========================================
  Files         260      254       -6     
  Lines       12624    12554      -70     
  Branches      848      807      -41     
==========================================
- Hits         9211     9192      -19     
+ Misses       3413     3362      -51     

Impacted Files	Coverage Δ
src/JsonRpc.Generators/Contexts/DapAttributes.cs	100.00% <ø> (ø)
...ors/Strategies/ExtensionMethodGeneratorStrategy.cs	100.00% <ø> (ø)
....Generators/Strategies/HandlerGeneratorStrategy.cs	14.51% <ø> (ø)
...nMethodGeneratorWithRegistrationOptionsStrategy.cs	100.00% <ø> (ø)
...tMethodGeneratorWithRegistrationOptionsStrategy.cs	48.11% <ø> (ø)
...thodGeneratorWithoutRegistrationOptionsStrategy.cs	100.00% <ø> (ø)
...eMethodGeneratorWithRegistrationOptionsStrategy.cs	1.85% <ø> (ø)
...thodGeneratorWithoutRegistrationOptionsStrategy.cs	0.00% <ø> (ø)
...ators/Strategies/TypedDelegatingHandlerStrategy.cs	2.81% <ø> (ø)
...erators/AssemblyCapabilityKeyAttributeGenerator.cs	1.56% <1.88%> (-2.24%)	⬇️
... and 26 more

---

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 9ddacc0...1bffa0c. Read the comment docs.