DXCDT-450: Remove client deprecated fields

[willvedd]

🔧 Changes

Ahead of our upcoming major release, this PR removing the deprecated client_secret and token_endpoint_auth_method properties from the auth0_client resource and put on the correlating data source instead.

📚 References

Migration Guide

🔬 Testing

Some tests changes to test new behavior changes and non-behavioral test changes to remove deprecated components.

📝 Checklist

• All new/changed/fixed functionality is covered by tests (or N/A)
• I have added documentation for all new/changed functionality (or N/A)

[codecov-commenter]

Codecov Report

Merging #725 (58b06f9) into v1 (5499aab) will decrease coverage by 0.05%.
The diff coverage is 100.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##               v1     #725      +/-   ##
==========================================
- Coverage   87.54%   87.49%   -0.05%     
==========================================
  Files          87       87              
  Lines       13244    13235       -9     
==========================================
- Hits        11594    11580      -14     
- Misses       1247     1251       +4     
- Partials      403      404       +1     

Impacted Files	Coverage Δ
internal/auth0/client/expand.go	87.84% <ø> (-0.02%)	⬇️
internal/auth0/client/data_source.go	85.91% <100.00%> (+5.52%)	⬆️
internal/auth0/client/flatten.go	85.20% <100.00%> (+1.70%)	⬆️
internal/auth0/client/resource.go	98.71% <100.00%> (-0.43%)	⬇️