Fix Jaeger propagator baggage header case sensitivity

[thswlsqls]

Description

• JaegerPropagator.getBaggageFromHeader() matched baggage keys case sensitively (uberctx-, jaeger-baggage), so carriers that return header keys in their original case (e.g. undertow 1.7 sends Uberctx-Key, Jaeger-Baggage) had their baggage silently dropped.
• Lowercase the carrier key with key.toLowerCase(Locale.ROOT) before matching the prefix and header name; the value lookup still uses the original key.
• Mirrors the sibling OtTracePropagator.extract(), which already lowercases the key (#5840). Jaeger was missed.

Testing done

• Added JaegerPropagatorTest#extract_baggageOnly_capitalizedHeaders covering both a capitalized Uberctx-* prefix and a capitalized Jaeger-Baggage header.
• ./gradlew :extensions:trace-propagators:check — 39 tests passed in JaegerPropagatorTest, full module check green.
• No public API change: private method, jApiCmp produced no apidiff change.
• Added a ## Unreleased entry in CHANGELOG.md.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.77%. Comparing base (824334c) to head (9ba35ce).

Additional details and impacted files

@@            Coverage Diff            @@
##               main    #8496   +/-   ##
=========================================
  Coverage     78.77%   78.77%           
- Complexity     8579     8580    +1     
=========================================
  Files          1009     1009           
  Lines         28993    28994    +1     
  Branches       3599     3599           
=========================================
+ Hits          22839    22841    +2     
  Misses         5311     5311           
+ Partials        843      842    -1     

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