opentelemetry-zpages publish fix - remove unsupported experimental category slug

[lalitb]

Changes

opentelemetry-zpages publishin fails with error:

   Uploading opentelemetry-zpages v0.9.0 (/home/labhas/obs/ot/lalit/rust/opentelemetry-rust-contrib/opentelemetry-zpages)
error: failed to publish to registry at https://crates.io

Caused by:
  the remote server responded with an error (status 400 Bad Request): The following category slugs are not currently supported on crates.io: experimental

  See https://crates.io/category_slugs for a list of supported slugs.`

Removing unsupported experimental category flag in this PR.

Merge requirement checklist

• CONTRIBUTING guidelines followed
• Unit tests added/updated (if applicable)
• Appropriate CHANGELOG.md files updated for non-trivial, user-facing changes
• Changes in public API reviewed (if applicable)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 55.7%. Comparing base (56a6f45) to head (cbba031).
Report is 20 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##            main     #91     +/-   ##
=======================================
+ Coverage   52.3%   55.7%   +3.4%     
=======================================
  Files         38      38             
  Lines       4967    5394    +427     
=======================================
+ Hits        2598    3006    +408     
- Misses      2369    2388     +19     

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