[ADMIN] Add Doug Barker to approvers

[dbarker]

Add Doug Barker (@dbarker) to approvers

Per #3220 (comment)

[netlify]

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

Name	Link
🔨 Latest commit	7db8703
🔍 Latest deploy log	https://app.netlify.com/sites/opentelemetry-cpp-api-docs/deploys/67e6e4e243c6350008572cf1

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.52%. Comparing base (a6779d8) to head (7db8703).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3331   +/-   ##
=======================================
  Coverage   89.52%   89.52%           
=======================================
  Files         210      210           
  Lines        6526     6526           
=======================================
  Hits         5842     5842           
  Misses        684      684           

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.