docs: remove references to default-engine feature

[kyli87]

What changes are proposed in this pull request?

The default-engine feature flag was removed in #1100 (several months ago) but the docs were never updated to reflect this. Instead, the feature flag should have an explicit suffix indicating which tls backend to use.

This PR just updates the docs to reflect this.

How was this change tested?

n/a - just docs

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.32%. Comparing base (9decd69) to head (c312f87).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2417   +/-   ##
=======================================
  Coverage   88.32%   88.32%           
=======================================
  Files         171      171           
  Lines       56727    56727           
  Branches    56727    56727           
=======================================
  Hits        50103    50103           
  Misses       4704     4704           
  Partials     1920     1920           

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[rliao147]

thanks for the change!