log: Introduce and use `NewLogMatcher`

We use log.DeciderOption to obtain a grpcmwlogrus.Option which is used to only apply logs to certain grpc methods. This option is no longer available in 'go-grpc-middleware@v2'.

To overcome this we introduce and use log.NewLogMatcher which satisfies the Matcher interface within 'go-grpc-middleware@v2' and then use this along with the selector.*ServerInterceptor to provide the same functionality.

We also deprecate the old code and use the new code, along with the tests. This provides a good opportunity to introduce a mock 'slog' to be used as an implementation of log.Logger in tests.

Part of #5405 (closed), #5661