Changed:Default trace propagation styles to Datadog,tracecontext

[marcotc]

For the release notes

This release changes the default trace propagation style from Datadog,b3multi,b3 to Datadog,tracecontext.
This aligns the Ruby APM with other APM languages, increasing cross-language compatibility out of the box.

This change also improve interoperability with the W3C tracecontext propagation standard, used by OpenTelemetry systems.

Unless you are using B3 propagation and are not configuring an explicit DD_TRACE_PROPAGATION_STYLE_EXTRACT (or tracing.distributed_tracing.propagation_extract_style), this change will not reduce observability.
Otherwise, if you use B3 propagation, you can set the environment variable DD_TRACE_PROPAGATION_STYLE_EXTRACT=Datadog,b3multi,b3 to restore the previous behavior.

For Datadog employees:

• If this PR touches code that signs or publishes builds or packages, or handles
  credentials of any kind, I've requested a review from @DataDog/security-design-and-guidance.
• This PR doesn't touch any of that.

[codecov-commenter]

Codecov Report

Merging #3248 (5bdf792) into master (8ef4139) will increase coverage by 0.00%.
Report is 51 commits behind head on master.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #3248   +/-   ##
=======================================
  Coverage   98.22%   98.22%           
=======================================
  Files        1253     1253           
  Lines       72303    72252   -51     
  Branches     3355     3355           
=======================================
- Hits        71019    70969   -50     
+ Misses       1284     1283    -1     

Files	Coverage Δ
lib/datadog/tracing/configuration/settings.rb	95.07% <100.00%> (ø)
spec/datadog/opentelemetry_spec.rb	100.00% <100.00%> (ø)
spec/datadog/opentracer/rack_propagator_spec.rb	100.00% <100.00%> (ø)
...pec/datadog/tracing/configuration/settings_spec.rb	100.00% <100.00%> (ø)
...ec/datadog/tracing/distributed/propagation_spec.rb	100.00% <100.00%> (ø)

... and 1 file with indirect coverage changes

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