[11.x] feat: use phpredis 6 in ci

[calebdw]

Hello!

This updates the ci tests workflow to use phpredis 6 instead of 5.7.3.

Thanks!

[GrahamCampbell]

I believe testing on 5.3.7 was intentional, to ensure we work with the older extension.

[calebdw]

@GrahamCampbell, I'm sure it was intentional---the real question is when to move to phpredis 6?

The phpredis 6 version has been out for almost a year and it the version that is installed when users execute:

pecl install phpredis

By moving to v6 this ensures that we continue to work with the newer version which is what users will be installing by default. If v6 was running in ci then this would have already been caught: #51346

However, I have no issues if y'all decide to stick with 5.7.3 and close this PR---I don't have the whole backstory on the decision to continue supporting 5.7.3, but I'm just trying to help :)