Update default version to 8.0

[stebogit]

MySQL 5.7 has officially reached EOL in Dec 2023, I believe the default version should now be 8.0.

I'd be happy to try and push a PR for this

[lefte]

After discussion with the contributors, in order to prevent a user from running a lando rebuild and "upgrading" their MySQL from 5.7 to 8.x, which would be a major breaking change, we will keep 5.7 as the "default" for now. Upcoming Lando version 4 services will introduce new concepts around "default" versions, which should alleviate this issue for almost all users.

Therefore, I recommend closing this issue as wont-fix for now.