Inconsistent diagnostic option names for command line and environment variable

[Bertk]

Describe the bug

Diagnostic option name was renamed from --diagnostic-output-fileprefix to --diagnostic-file-prefix (#6165) but the environment variable name is still TESTINGPLATFORM_DIAGNOSTIC_OUTPUT_FILEPREFIX.

https://github.com/dotnet/docs/blob/fde0a56ed2df06307faec6bfa9fba7ea7e687f02/docs/core/testing/microsoft-testing-platform-migration-from-v1-to-v2.md?plain=1#L70

This problem also exists for --diagnostic-synchronous-write option (former --diagnostic-filelogger-synchronouswrite).

The documentation is still using the old option name:
https://github.com/dotnet/docs/blob/main/docs/core/testing/microsoft-testing-platform-extensions-diagnostics.md

Steps To Reproduce

Check code for diagnostic extension and documentation:

testfx/src/Platform/Microsoft.Testing.Platform/CommandLine/PlatformCommandLineProvider.cs

Line 18 in 4e84b84

public const string DiagnosticOutputFilePrefixOptionKey = "diagnostic-file-prefix";

testfx/src/Platform/Microsoft.Testing.Platform/Helpers/EnvironmentVariableConstants.cs

Line 23 in 4e84b84

public const string TESTINGPLATFORM_DIAGNOSTIC_OUTPUT_FILEPREFIX = nameof(TESTINGPLATFORM_DIAGNOSTIC_OUTPUT_FILEPREFIX);

https://github.com/dotnet/docs/blob/fde0a56ed2df06307faec6bfa9fba7ea7e687f02/docs/core/testing/microsoft-testing-platform-extensions-diagnostics.md?plain=1#L21

https://github.com/dotnet/docs/blob/fde0a56ed2df06307faec6bfa9fba7ea7e687f02/docs/core/testing/microsoft-testing-platform-extensions-diagnostics.md?plain=1#L31

Expected behavior

documentation and environment variable name use identical name e.g. environment name: TESTINGPLATFORM_DIAGNOSTIC_FILE_PREFIX

Actual behavior

names are mixed up in documentation and implementation

[mariam-abdulla]

We are not planning to update the env var names in the meantime as this is a breaking change.
But we will update the documentation to reflect the new/correct option names.