[6.x] Fix phpredis "zadd" and "exists" on cluster

[themsaid]

This PR fixes #29637 and #30473

By not using executeRaw, the cluster connection will call the correct redis command.

[stancl]

@themsaid

Removing exists() is a breaking change, because the parameters are now different. Previously passing multiple arguments was okay, but now only one argument can be passed.

My package's tests were passing and now they are failing because of that. See this: https://travis-ci.com/github/stancl/tenancy/builds/152720978#L2193

And this: https://github.com/stancl/tenancy/blob/2.x/src/StorageDrivers/RedisStorageDriver.php#L59-L61

[themsaid]

@stancl but I can pass multiple keys no problem. Maybe you’re using a really old phpredis version?

[stancl]

@themsaid Hmm, possible. Seems like I'm using 3.1.6 in that build and the latest version is 5.2.0.

I guess I can find a way to update that, but I wonder if this would cause issues for others as well, since (for example) my code is a package.

But I see that you specified ^4.0 as phpredis version constraint, so I'll try with phpredis 4.