Move over to using VS Code's log level support

[brettcannon]

This lets us drop our custom setting and support and instead rely on what's built into VS Code.

[karthiknadig]

This requires following work:

• Switch the extension to use new logging channel: Use new logging API for python extension logger and LS logger #21062
• Preserve output channel for test output, since it is actual output and not log: Use new logging API for python extension logger and LS logger #21062
• Deprecate settings for logging in favor of VS code.