Case-insensitive propagation style configuration

[marcotc]

2.0 Upgrade Guide notes
The the values for the environment variables DD_TRACE_PROPAGATION_STYLE, DD_TRACE_PROPAGATION_STYLE_INJECT, and DD_TRACE_PROPAGATION_STYLE_EXTRACT are now considered case-insensitive.
The major impact of this change is that, previously, B3 would configure Tracing with the B3 Multiple Headers propagator. Now B3 will configure it with the B3 Single Header propagator.

Motivation:

This change aligns the Ruby tracer with all other Datadog Tracers.

Additional Notes:

How to test the change?

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.

Unsure? Have a question? Request a review!

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (9ef2354) 97.99% compared to head (d4b1d76) 97.99%.

Additional details and impacted files

@@                    Coverage Diff                    @@
##           remove-old-style-envs    #3299      +/-   ##
=========================================================
- Coverage                  97.99%   97.99%   -0.01%     
=========================================================
  Files                       1248     1248              
  Lines                      72011    72028      +17     
  Branches                    3392     3392              
=========================================================
+ Hits                       70570    70586      +16     
- Misses                      1441     1442       +1     

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