Enable WITH_OTLP_GRPC_SSL_MTLS_PREVIEW by default

[marcalff]

Fixes # (issue)

Changes

Please provide a brief description of the changes here.

• Enable WITH_OTLP_GRPC_SSL_MTLS_PREVIEW by default

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

• CHANGELOG.md updated for non-trivial changes
• Unit tests have been added
• Changes in public API reviewed

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.18%. Comparing base (a6e9a32) to head (b7a7dcd).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3970   +/-   ##
=======================================
  Coverage   90.18%   90.18%           
=======================================
  Files         230      230           
  Lines        7298     7298           
=======================================
  Hits         6581     6581           
  Misses        717      717           

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[dbarker]

Looks good. Thanks!