fix(exporter-prometheus): add missing semantic-conventions dependency

[omizha]

Which problem is this PR solving?

The withoutScopeInfo feature added in #5993 imports ATTR_OTEL_SCOPE_NAME and ATTR_OTEL_SCOPE_VERSION from
@opentelemetry/semantic-conventions at runtime, but the package was only listed in devDependencies.

This causes module resolution errors when using the exporter in projects with strict dependency resolution (e.g.,
Yarn PnP):

Error: @opentelemetry/exporter-prometheus tried to access @opentelemetry/semantic-conventions,
but it isn't declared in its dependencies

Short description of the changes

Move @opentelemetry/semantic-conventions from devDependencies to dependencies in
experimental/packages/opentelemetry-exporter-prometheus/package.json.

Type of change

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

How Has This Been Tested?

• All existing unit tests pass (62 passing)

npm run test -w @opentelemetry/exporter-prometheus

62 passing (68ms)

Checklist:

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

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: omizha / name: Haian (666373b, 9c74389)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.59%. Comparing base (0fd543c) to head (666373b).
⚠️ Report is 9 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6330   +/-   ##
=======================================
  Coverage   95.58%   95.59%           
=======================================
  Files         314      314           
  Lines        9590     9596    +6     
  Branches     2221     2221           
=======================================
+ Hits         9167     9173    +6     
  Misses        423      423           

see 2 files with indirect coverage changes

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[otelbot]

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