feat: use redis:7 instead of redis:6-bullseye, move Redis config, for #17

[stasadev]

The Issue

We used old redis:6-bullseye only to support older systems.

• Use redis:6-bullseye to support old systems, update GitHub test workflow #17

This change was done almost 2 years ago, it's time to update.

How This PR Solves The Issue

• Uses redis:7 by default.
• Moves Redis config to the same location as in ddev/ddev-redis-7
• People still can use redis:6-bullseye if they set it explicitly.

Manual Testing Instructions

ddev add-on get https://github.com/ddev/ddev-redis/tarball/20250425_stasadev_redis_7
ddev restart

# check for redis 7
ddev redis-cli INFO

Automated Testing Overview

Release/Deployment Notes