fix(redis): Require version >=4 for unlink

[solracsf]

Summary

v4+ (released 7y ago...) is required for unlink

server/lib/private/Memcache/Redis.php

Line 79 in 86f5fb0

if ($this->getCache()->unlink($this->getPrefix() . $key)) {

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)

[solracsf]

/backport to stable29

[solracsf]

/backport to stable28

[nickvergessen]

Should add that to https://docs.nextcloud.com/server/latest/admin_manual/configuration_server/caching_configuration.html ?

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