Update spec-compliance-matrix.md for C++

[marcalff]

Changes

Update spec-compliance-matrix.md for C++:

• Get a Tracer with scope attributes implemented by:
  • [API] Add InstrumentationScope attributes in TracerProvider::GetTracer() opentelemetry-cpp#2371
• Links can be recorded after span creation implemented by:
  • [API] Add a new AddLink() operation to Span opentelemetry-cpp#2380
• get_meter accepts attributes implemented by:
  • [API] Add InstrumentationScope attributes in MeterProvider::GetMeter() opentelemetry-cpp#2224

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
• spec-compliance-matrix.md updated if necessary

[carlosalberto]

A reminder for the @open-telemetry/cpp-approvers that AddLink() currently is an experimental feature (which means it still may change - hopefully not)