Use correct type for severity_logger

[bergjohan]

Steps to reproduce Create a severity_logger with the type of boost::log::trivial::severity_level instead of int.

boost::log::sources::severity_logger<boost::log::trivial::severity_level> logger;
BOOST_LOG_SEV(logger, boost::log::trivial::info) << "Test message with severity";

What is the expected behavior?

  severity_num       : 9
  severity_text      : INFO
  body               : Test message with severity

What is the actual behavior?

  severity_num       : 0
  severity_text      : INVALID
  body               : Test message with severity

Additional context boost::log::extract_or_default fails here, because it expects the type of int: https://github.com/open-telemetry/opentelemetry-cpp-contrib/blob/main/instrumentation/boost_log/src/sink.cc#L71

We are clearly using boost::log::trivial::severity_level by default (https://github.com/open-telemetry/opentelemetry-cpp-contrib/blob/main/instrumentation/boost_log/include/opentelemetry/instrumentation/boost_log/sink.h#L60), so why not use the correct type everywhere?