Display a warning when users request unsupported JRE versions

[CAFxX]

Since anything below JRE 1.8 is unsupported, we think it would be beneficial to show a warning during staging when users request a JRE version that is known to be unsupported.
While JRE <1.8 being EoL should be a well-known fact, supporting EoL versions with no warnings can give users the impression that those versions are still supported by the platform.

[nebhale]

Yep, that's a great idea. I'll make sure it makes it into the backlog quickly.