Update the MySQL version used in the Docker lite images

[frouioui]

The MySQL version we are using in our vitess/lite images is pretty old (8.0.23) we should upgrade it to a more recent and stable version: 8.0.31.

• Find the implications of this change. Run some upgrade/downgrade tests between v15 and v16 on docker. and also with the operator.
• Use those findings to warns users in the release notes.
• We also need to change the default docker image we publish from 5.7 to 8.0.

[deepthi]

We also need to change the default docker image we publish from 5.7 to 8.0.

[frouioui]

For reference: linking #7812 to this issue as the enhancement proposed by the two issues is the same.