Stabilize logger.enabled by removing spec_unstable_logs_enabled feature flag

[Copilot]

This PR stabilizes the logger.enabled functionality by completely removing the spec_unstable_logs_enabled feature flag from all OpenTelemetry Rust crates as requested in the issue.

Changes Made

Core Crates Updated

• opentelemetry: Removed feature flag definition and conditional compilation
• opentelemetry_sdk: Removed feature flag definition and conditional compilation
• opentelemetry-appender-log: Removed feature flag definition and conditional compilation
• opentelemetry-appender-tracing: Removed feature flag definition and conditional compilation

Key Functional Changes

• Logger::event_enabled() method is now always available (previously behind feature flag)
• LogProcessor::event_enabled() trait method is now always available
• LogExporter::event_enabled() trait method is now always available
• Log level control functionality is now enabled by default in all log appenders

Documentation & Cleanup

• Updated CHANGELOG.md files for all affected crates
• Removed documentation references to the feature flag
• Updated build scripts and benchmarks to remove feature flag references
• Cleaned up dependencies in stress tests and examples

Breaking Change Notice

This is a breaking change for users who were explicitly enabling the spec_unstable_logs_enabled feature flag. However, the functionality remains identical - it's just always enabled now.

Before:

opentelemetry = { version = "0.30", features = ["logs", "spec_unstable_logs_enabled"] }

After:

opentelemetry = { version = "0.30", features = ["logs"] }

The logger.enabled functionality that was previously experimental is now stable and ready for production use.

Verification

• ✅ All core crates build successfully
• ✅ All appender tests pass
• ✅ Logger.enabled functionality works correctly
• ✅ No remaining references to the feature flag (except in historical CHANGELOGs)

Fixes #3020.

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[linux-foundation-easycla[bot]]

• :white_check_mark: login: cijothomas / name: Cijo Thomas (13161050b05a217ed8fa0bd2c713acdaeba89a96)
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[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 80.0%. Comparing base (af2f144) to head (1316105). Report is 8 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #3036     +/-   ##
=======================================
- Coverage   81.1%   80.0%   -1.1%     
=======================================
  Files        126     126             
  Lines      24954   21879   -3075     
=======================================
- Hits       20251   17519   -2732     
+ Misses      4703    4360    -343     

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