Drop switch from Redis unlink to delete

[michaldo]

Drop support for Redis 3, which is not maintained since 2018

[artembilan]

After discussion with Spring Data team, we agreed that fully dropping DEL support is fair game from now on.
Please, consider to refine the logic just deal with UNLINK.
Also, take a look into RedisQueueInboundGateway, RedisQueueMessageDrivenEndpoint and RedisMessageStore where we still use DEL.

Thanks

[michaldo]

I decided to remove switch from unlink to dev
About places where del is used, I will take a look later

[artembilan]

I will take a look later

Thank you!

Let's us know, please, if you are going to that as a part of this PR or so!
In the end we would need to mention in the whats-new.adoc such a change in Redis module.
Also, the redis.adoc need to be revised if we still mention DEL somewhere there and probably an algorithm you've just dropped.

[artembilan]

Also, just noticed:

Signed-off-by: michaldo <michaldo@github.com>

Please, use your legal name instead according to DCO requirements.
You can configure your Git client to produce a proper name for you into those commits.

[michaldo]

This PR is functionally closed. I encountered problem of not careful switch, analyzed it and fixed. I am sure this change is safe because no modern application should be switched to delete mode.
Regarding other places where delete is used, I'm not familiar with context. I'm going to take a look.

[artembilan]

Please, add your name into the @author list of the affected class.
Also, give us your thought about other classes in Redis module which still use DEL.

Thank you again!

[michaldo]

I add myself as one of @author's
Regarding other classes:

• RedisMessageStore: is it same case switch from unlink -> delete and I removed it also

• RedisQueueInboundGateway, RedisQueueMessageDrivenEndpoint: they use delete command within JMX @ManagedOperation command. I feel that JMX action looks like admin action and I think If were someone who executes admin action I prefer to be sure that when action is completed everything is done. It mean I prefer synchronous delete over asynchronous unlink

[artembilan]

I prefer to be sure that when action is completed everything is done

Sounds reasonable.
That was oversight on my side when I performed search for RedisTemplate.delete() 😄

Thank you for looking over there!