Add additional information to the "Install the application…" section of magento2.md

[MylesForrest]

Adds a bit more information about which Adobe Commerce versions expect non-redis cache and session flags, as well as a link to the Experience League page describing how to configured Valkey.

The linked page also includes additional information about affected versions.

[MylesForrest]

Hope this isn't a ridiculous change - at first I made a second section that contained another example install script with the correct flags, but that made the example install script section unwieldy.

🤷

[navarr]

Thanks Myles! I was just thinking about this the other day when updating some redis to valkey hostnames.

[MylesForrest]

No problem - I was setting up a 2.4.8 env and after my third attempt at running the install, realized that on 2.4.8 you can be in the awkward position of using "redis" flags for valkey figured it was worth noting