[RELEASE] Release opentelemetry-cpp 1.20.0

[marcalff]

Fixes #3291

Changes

Release opentelemetry-cpp 1.20.0

For significant contributions please make sure you have completed the following items:

• CHANGELOG.md updated for non-trivial changes
• Unit tests have been added
• Changes in public API reviewed

[marcalff]

Waiting for:

• [METRICS SDK] Fix hash collision in MetricAttributes #3322

[netlify]

✅ Deploy Preview for opentelemetry-cpp-api-docs canceled.

Name	Link
🔨 Latest commit	45a6db2
🔍 Latest deploy log	https://app.netlify.com/sites/opentelemetry-cpp-api-docs/deploys/67ec5774748602000a36d3fb

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.55%. Comparing base (3bd8de9) to head (45a6db2).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3328   +/-   ##
=======================================
  Coverage   89.55%   89.55%           
=======================================
  Files         210      210           
  Lines        6502     6502           
=======================================
  Hits         5822     5822           
  Misses        680      680           

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