Use Polysharp to take advantage of latest language features

[kailash-b]

Changes

• Add dependency on Polysharp library v1.15.0
• Use latest language features in all projects
• Use Null Reference Types in Auth0.Core Project

References

• SDK-5405
• Support Nullable Reference Types #750

Testing

All unit tests and integration tests are passing.

Checklist

• I have read the Auth0 general contribution guidelines

• I have read the Auth0 Code of Conduct

• All existing and new tests complete without errors

[codecov]

Codecov Report

Attention: Patch coverage is 94.73684% with 1 line in your changes missing coverage. Please review.

Project coverage is 74.96%. Comparing base (dbcec6d) to head (6a919ec).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
src/Auth0.Core/Exceptions/RateLimit.cs	80.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #813      +/-   ##
==========================================
- Coverage   74.98%   74.96%   -0.02%     
==========================================
  Files         444      444              
  Lines        5805     5805              
  Branches      363      363              
==========================================
- Hits         4353     4352       -1     
  Misses       1296     1296              
- Partials      156      157       +1     

Flag	Coverage Δ
authIntTests	30.88% <21.05%> (ø)
mgmtIntTests	57.01% <52.63%> (ø)
unittests	5.94% <89.47%> (-0.03%)	⬇️

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