Update the spec compliance matrix for C++.

[marcalff]

Update the spec compliance matrix for C++.

Changes

Please provide a brief description of the changes here.

• Logs section
  • Document that LoggerProvider.Shutdown and LoggerProvider.ForceFlush are implemented.
  • see https://github.com/open-telemetry/opentelemetry-cpp/blob/6445819fceeb3bfea1444a88418f00a0df69cd2f/sdk/include/opentelemetry/sdk/logs/logger_provider.h#L93-L101
  • Document that the OTLP File exporter is supported.
  • see https://github.com/open-telemetry/opentelemetry-cpp/blob/6445819fceeb3bfea1444a88418f00a0df69cd2f/exporters/otlp/include/opentelemetry/exporters/otlp/otlp_file_exporter.h#L26
• Context Propagation section
  • Document that TextMapPropagator is supported.
  • see https://github.com/open-telemetry/opentelemetry-cpp/blob/6445819fceeb3bfea1444a88418f00a0df69cd2f/api/include/opentelemetry/context/propagation/text_map_propagator.h#L42
• Environment Variables section
  • Many environment variables are still not supported in C++.
  • Document them explicitly with -, which is more informative than a blank.

For non-trivial changes, follow the change proposal process.

• [ ] Related issues #
• [ ] Related OTEP(s) #
• [ ] Links to the prototypes (when adding or changing features)
• [ ] CHANGELOG.md file updated for non-trivial changes
• [X] spec-compliance-matrix.md updated if necessary