sshkey 4.0

[bensie]

• Require Ruby 3.3+ / JRuby 10+
• CI: test 3.3, 3.4, 4.0, head, JRuby 10.0/10.1/head; actions/checkout v7; Dependabot for actions
• No runtime dependencies (drop base64)
• Ed25519: generate, load, and all outputs
• Load and write unencrypted OpenSSH format private keys; new #openssh_private_key
• SSHKey.new accepts an OpenSSL::PKey object
• ECDSA supported on JRuby; remove JRuby workarounds and OpenSSL::PKey::EC monkey patch
• generate requires type:; keyword arguments for generate and new
• RSA default 3072 bits; DSA generation removed (loading still supported)
• encrypted_private_key uses PKCS#8 (PBKDF2, AES-256)
• randomart defaults to SHA256 and matches ssh-keygen -lv
• Error hierarchy: SSHKey::Error, PrivateKeyError, PublicKeyError, UnsupportedError; ArgumentError for bad options
• Remove fingerprint and rsa_/dsa_ aliases, format_sshfp_record; make internal constants private
• Fix: no terminal passphrase prompt; "PRIVATE" in a public key comment; DSA/Ed25519 bit length
• Split tests by area; add CHANGELOG with upgrade guide; README updates; gemspec metadata

Closes #41