[chore] Update C++ spec compliance matrix

[dbarker]

Updates the C++ spec compliance matrix.

Changes

Update the C++ matrix to show support for the following features:

1. Associate Tracer with InstrumentationScope
   • [SDK] Fix include instrumentation scope attributes in equal method opentelemetry-cpp#3214
2. Gauge instrument is supported
   • [API] Add synchronous gauge opentelemetry-cpp#3029
3. Exponential Histogram Aggregation
   • [SDK] Base2 exponential histogram aggregation opentelemetry-cpp#3346
4. A valid instrument must be created when given the same name and duplicates result in the first seen instrument
   • [SDK] support aggregation of identical instruments opentelemetry-cpp#3358

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
  • For trivial changes, include [chore] in the PR title to skip the changelog check
• Spec compliance matrix updated if necessary

[dbarker]

@open-telemetry/cpp-approvers Please review. Thanks!