[RELEASE] Prepare release opentelemetry-cpp 1.21.0

[ThomsonTan]

No planned date, aiming for week 21

Very desirable / Must have:

• Fix lifetime for sdk::ReadWriteLogRecord #3147 or [EXPORTER] ostream log exporter, fix memory ownership issues #3417
• [BUILD] Propagate INTERFACE_COMPILE_DEFINITIONS from API through common_foo_library #3440

Desirable:

• [SDK] Fix Base2ExponentialHistogramAggregation Merge with empty buckets #3425
• [EXPORTER] support unix sockets in grpc client #3410
• [SEMANTIC CONVENTIONS] Fix comment style to preserve markup #3444

Deferred:

• (pending bazel central) [BUILD] Upgrade to opentelemetry-proto 1.7.0 #3443

[ThomsonTan]

@marcalff do you think we could target the new release in the next week?

[owent]

Is it possiable to merge #3147 ?