Prepare 26.4.0 release (#1527)

Raises the maximum supported cryptography version to 50.x and bumps
the version to 26.4.0.

Fixes https://github.com/pyca/pyopenssl/issues/1525
Closes https://github.com/pyca/pyopenssl/pull/1526


Claude-Session: https://claude.ai/code/session_01Vb6YjYEyUrAMT2UAQtaUHp

Co-authored-by: Claude <noreply@anthropic.com>