Verify Moodle works / passes ok using Redis 5 extension

Redis 5.0.0 was released few days ago:

https://pecl.php.net/package-info.php?package=redis&version=5.0.0

And it includes some changes that, at very least, are breaking our redis cachestore tests:

https://ci.moodle.org/job/S35.01.03%20-%20PHPUnit%20-%20MariaDB%20(lowest)/344/

Curiously, it doesn't break our redis session tests, only the cachestore ones, apparently because of the way we connect in those tests (with NULL being used as port).

So we are keeping our docker images working (sticky) with redis 4.3:

https://github.com/moodlehq/moodle-php-apache/pull/65

till we complete this, that will be in charge of:

• Verifying if the new version causes real problems to core or no. Fix them whenever needed.
• Making all tests to be back to pass.
• Unpinning the 4.3.0 PR above (all branches having it).

Ciao