Add issues to the TODOs in S.T.Json source for better tracking and minor clean up

[ahsonkhan]

• Remove unnecessary check in condition:

  • System.Text.Json - Use MaxExpansionFactorWhileTranscoding instead of counting bytes corefx#42574 (comment)
  • https://github.com/dotnet/corefx/pull/39541/files#diff-e2e1b5ba65b1a8b77a086eb93e77c2d1R327

• Add issues to the TODOs for better tracking and fix some that were easy and could be addressed quickly.

  • Add tests with missing object property and missing collection property. #32348
  • Update tests to reflect the different rounding/e-notation when writing numbers between S.T.J and Newtonsoft.Json #32350
  • Create new AssertContentsAgainstJsonNet to avoid calling NormalizeToJsonNetFormat when not necessary. #32351
  • Investigate caching the underlying internal converters like KeyValuePairConverter #32352
  • When handling $ref metadata verify value is converter.TypeToConvert and throw JsonException #32354
  • Investigate switching to ArrayBufferWriter impl to simplify grow logic when reading from stream in JsonSerializer #32355
  • Figure out the best flush threshold for serializer based on performance and real-world use cases #32356
  • Consider using the converters and JsonClassInfo from s_defaultOptions by cloning #32357
  • When initializing WriteStackFrame with re-entry during serialization, consider doing a type comparison #32358

• Update ActiveIssue and enable tests that are now fixed.

  • Consider tracking JSON path while deserializing JsonElement to return better JsonException #32359

[jozkee]

Otherwise, looks good.

[ahsonkhan]

CI failure is unrelated: #32367