use system OpenSSL and Docker

[drslebedev]

Use system OpenSSL for QUIC deps

• Drop the custom OpenSSL ExternalProject build and link QUIC components against system OpenSSL::SSL / OpenSSL::Crypto.

Update CI configuration to use Docker

• switch to clang-20

Use system nghttp2, while keeping ngtcp2/nghttp3 as ExternalProjects (ngtcp2_crypto_ossl isn’t packaged).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 49.06%. Comparing base (750cd50) to head (52d2492).
⚠️ Report is 27 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #399      +/-   ##
==========================================
- Coverage   56.84%   49.06%   -7.78%     
==========================================
  Files          72       78       +6     
  Lines        7561     9870    +2309     
  Branches     2838     3727     +889     
==========================================
+ Hits         4298     4843     +545     
- Misses       1335     2865    +1530     
- Partials     1928     2162     +234     

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[sonarqubecloud]

Quality Gate failed

Failed conditions
5 Security Hotspots
0.0% Coverage on New Code (required ≥ 80%)

See analysis details on SonarQube Cloud