Consistent naming standard for private static fields

[JamesNK]

dotnet/aspnetcore mixes _underscore and PascalCase for private static fields. We should pick one for consistency and enable a naming rule to enforce it.

Search results: https://github.com/search?q=repo%3Adotnet%2Faspnetcore%20%22private%20static%20readonly%22&type=code

Note: dotnet/runtime uses s_ prefix.

[mkArtakMSFT]

Potentially related then #50854.
Is there more work to be done here, @JamesNK ?

Thanks for contacting us.

We're moving this issue to the .NET 9 Planning milestone for future evaluation / consideration. We would like to keep this around to collect more feedback, which can help us with prioritizing this work. We will re-evaluate this issue, during our next planning meeting(s).
If we later determine, that the issue has no community involvement, or it's very rare and low-impact issue, we will close it - so that the team can focus on more important and high impact issues.
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[JamesNK]

Potentially related then #50854. Is there more work to be done here, @JamesNK ?

That PR doesn't have a rule for private static fields.