[Cache] Document Redis auth option and DSN username/auth parameters

[ckrack]

This PR documents the previously undocumented auth connection option for the Redis cache adapter and improves the DSN authentication examples.

Changes:

• Mention username support in the DSN description text
• Update DSN example to use a custom username instead of default
• Add example for auth[] query parameter syntax
• Document the auth option in the options array and the "Available Options" section
• Add note that Predis ignores the auth option (credentials must be provided via DSN)

Context:

This was requested in the discussion on symfony/symfony#63324, where it was pointed out that the auth parameter was undocumented.

[nicolas-grekas]

Cool thanks.
Ppl are also looking for differentiating sentinel vs master auth, see symfony/symfony#62218
Let's hint about this use case!

[MrYamous]

I cannot add a suggestion on a line which is not updated, but I think we could use an before username

[nicolas-grekas]

Let's account for symfony/symfony#63391 here also, and likely update the messenger doc too. 🙏

[ckrack]

Let's account for symfony/symfony#63391 here also, and likely update the messenger doc too. 🙏

Should i target 7.4 then?

[nicolas-grekas]

Ah right, yes, 7.4 is good enough