Remove support for native-tls

[mgoldenberg]

Note: this pull request should be merged in conjunction with matrix-org/complement-crypto#236.

This pull request removes both the native-tls and rustls-tls feature flags in all crates where they exist. The resulting behavior is that rustls becomes the only TLS backend, and no others can be selected.

The benefit here is that rustls supports TLS v1.3 on all platforms, whereas native-tls does not (see #6053). Additionally, the native-tls and rustls-tls feature flags were not additive, but rather they were mutually exclusive. This is not an ideal property for feature flags.

From The Cargo Book

...features should be additive. That is, enabling a feature should not disable functionality, and it should usually be safe to enable any combination of features.

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Closes #6383.

• I've documented the public API Changes in the appropriate CHANGELOG.md files.
• This PR was made with the help of AI.

Signed-off-by: Michael Goldenberg m@mgoldenberg.net

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.85%. Comparing base (a65fec3) to head (ab83ed0).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6409      +/-   ##
==========================================
- Coverage   89.87%   89.85%   -0.02%     
==========================================
  Files         378      378              
  Lines      103445   103445              
  Branches   103445   103445              
==========================================
- Hits        92973    92953      -20     
- Misses       6902     6915      +13     
- Partials     3570     3577       +7     

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[codspeed-hq]

Merging this PR will not alter performance

✅ 50 untouched benchmarks

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_{Comparing mgoldenberg:remove-native-tls (ab83ed0) with main (a65fec3)}