Override find_cert_chain instead of the deprecated cert_chain in the TLS examples

[moritzschmitt]

The windows-11-arm runners are being migrated to the Visual Studio 2026 image this week (actions/runner-images#14602). Its MSVC 19.51 emits C4996 when a class overrides a [[deprecated]] virtual function, and with --werror-mode the amalgamation build fails in tls_client.cpp, tls_proxy.cpp and tls_ssl_key_log_file.cpp, which override Credentials_Manager::cert_chain:

src/examples/tls_client.cpp(46): warning C4996: 'Botan::Credentials_Manager::cert_chain': Do not define or use this function; use find_cert_chain

Seen first on #5941, where the same tree passed with MSVC 19.44 a few hours earlier. The library and the tests are not affected because BOTAN_IS_BEING_BUILT disables BOTAN_DEPRECATED.

The three examples now override find_cert_chain instead, which is what the deprecation message asks for. Behaviour is unchanged; the new acceptable_CAs parameter is ignored like the others.

[reneme]

Thank you.

[randombit]

Thanks