feat: 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]

• ✅ login: cijothomas / name: Cijo Thomas (1316105)
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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

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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[cijothomas]

@copilot The changelog make it sound like this is a breaking change to crate that is announced as stable. Given the feature flag was already marked "unstable", please modify changelog so that it'll not scare regular consumers of the crate with breaking change warning.

[Copilot]

Updated the changelog to remove the "Breaking" label and clarify this was removing an experimental feature flag. The entry now reads "Remove experimental spec_unstable_logs_enabled feature flag - logger.enabled functionality is now stabilized and always available." (commit 8b8c805)