devp2p: drop pre-EIP8 support, fix snappy negotiation

[arnetheduck]

Support for previous versions was removed from geth in 2021 after other clients had migrated - should be safe to remove here also.

• fix snappy detection - it should use the hello version, not the RLPx handshake
• simplify generation of auth messages
• pad 100-300 bytes like spec suggests

TODO: error handling is all over the place - will be addressed in a follow-up PR