Update Redis stub from an very outdated version to the latest stable version (6.0.2)

[karstennilsen]

The redis stub contains:

@link https://github.com/phpredis/phpredis/blob/develop/redis.stub.php

But the stub itself was not even close to similar / up to date with the linked version.
This is causing unexpected situations where e.g. the ttl() method in reality returns -2 when a key is not found but the old stub from this project specified false.

[karstennilsen]

FYI: https://redis.io/docs/latest/commands/ttl/ states In Redis 2.6 or older the command returns -1 if the key does not exist or if the key exist but has no associated expire.

So the current stub is referencing an implementation of Redis 2.6 or older. Redis 2.6 last release was in 2013. You can't even find it anymore in the Redis.com download archive.