chore(api): mark ProxyTracerProvider as deprecated

[cjihrig]

Which problem is this PR solving?

ProxyTracerProvider should not be exported from the API.

Fixes #6321

Short description of the changes

This commit completes the checklist for ProxyTracerProvider in #6321.

Type of change

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

• Test suite

Checklist:

• Followed the style guidelines of this project
• Unit tests have been added
• Documentation has been updated

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.58%. Comparing base (e5f2d42) to head (835d59b).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6328   +/-   ##
=======================================
  Coverage   95.58%   95.58%           
=======================================
  Files         314      314           
  Lines        9590     9590           
  Branches     2221     2221           
=======================================
  Hits         9167     9167           
  Misses        423      423           

Files with missing lines	Coverage Δ
api/src/trace/ProxyTracerProvider.ts	85.71% <ø> (ø)

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[cjihrig]

The CI failures appear to be GitHub infra issues.

[otelbot]

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