[BUILD] Remove the hard-coded separator in tracestate

[ThomsonTan]

Changes

Please provide a brief description of the changes here.

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

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 87.59%. Comparing base (497eaf4) to head (74cd28e).
Report is 64 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2672      +/-   ##
==========================================
+ Coverage   87.12%   87.59%   +0.47%     
==========================================
  Files         200      188      -12     
  Lines        6109     5847     -262     
==========================================
- Hits         5322     5121     -201     
+ Misses        787      726      -61     

Files	Coverage Δ
api/include/opentelemetry/trace/trace_state.h	97.60% <100.00%> (ø)

... and 57 files with indirect coverage changes

[marcalff]

Thanks for the fix.