Rename Baggage Header name to baggage

[utpilla]

Fixes #1967

Changes

• Renamed BaggageHeader name to baggage (lowercase). BaggagePropagator.Extract and BaggagePropagator.Inject will now look for and add baggage to the carrier respectively
• Updated HttpInstrumentationTests to look for baggage in Headers instead of Baggage. Although these look ups are case insensitive, I updated them to be consistent with the BaggageHeader Name.

For significant contributions please make sure you have completed the following items:

• CHANGELOG.md updated for non-trivial changes

[reyang]

LGTM once the CHANGELOG.md is updated.

[codecov]

Codecov Report

Merging #2003 (a39ba97) into main (f3912e5) will increase coverage by 0.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #2003      +/-   ##
==========================================
+ Coverage   83.87%   83.91%   +0.03%     
==========================================
  Files         192      192              
  Lines        6184     6184              
==========================================
+ Hits         5187     5189       +2     
+ Misses        997      995       -2     

Impacted Files	Coverage Δ
...metry.Api/Context/Propagation/BaggagePropagator.cs	83.92% <ø> (ø)
...Telemetry/Internal/SelfDiagnosticsEventListener.cs	96.58% <0.00%> (+0.85%)	⬆️
...ZPages/Implementation/ZPagesExporterEventSource.cs	62.50% <0.00%> (+6.25%)	⬆️