[yegorich]

The openssl job uses Ubuntu's libssl-dev, which is OpenSSL 3.0, so CI doesn't cover OpenSSL 4.x.
With #36, libuhttpd builds against OpenSSL 4.0, so this adds a matrix entry openssl4:

• OpenSSL 4.0.3: downloaded from the official release and checked against its SHA-256. It's built
  as shared libraries, without docs and tests, and installed to ~/openssl.
• Caching: the installation is cached with actions/cache, keyed by version, so it's only built
  once per version.
• libuhttpd: built with -DUSE_OPENSSL=ON and PKG_CONFIG_PATH pointing to that installation.
• Guard: the job fails if src/libuhttpd.so doesn't link libssl.so.4. Without it, a broken
  installation would let CMake fall back to the system OpenSSL 3, and the job would pass without
  testing 4.x.

The other jobs are unchanged. Updating to a later 4.x release means changing openssl_version and
openssl_sha256. rtty's CI has the same kind of job for Mbed TLS 4.x.

Testing

I ran the steps in an ubuntu:24.04 container, each with bash -eo pipefail as GitHub Actions does:

• OpenSSL 4.0.3 downloads, passes the hash check, and builds.
• CMake reports Found openssl, version 4.0.3, libuhttpd builds without warnings, and libuhttpd.so
  links libssl.so.4.
• The existing openssl job still builds against OpenSSL 3.0.13.
• With the OpenSSL 4.0.3 installation removed, CMake falls back to the system OpenSSL 3.0.13, and the
  guard fails the step as intended.

The workflow also passes on my fork, with all five jobs green. The openssl4 job built OpenSSL and
saved the openssl-4.0.3-ubuntu-24.04 cache:
https://github.com/yegorich/libuhttpd/actions/runs/38147582273