CMake: Require and use Java 1.8

[davidebeatrici]

The specific version is mentioned in the build instructions at: https://github.com/MultiMC/Launcher/blob/abc582466ee7a88a398604c11b8ccb6be424c1fe/BUILD.md

Without this patch compilation fails because target and source have a minimum value of 8.

[davidebeatrici]

I just realized there is an identical commit in PolyMC: PolyMC/PolyMC@8e842d1

[phit]

why would you want to drop support for java 7? theres plenty of old mods that do not work on java 8

[Moresteck]

why would you want to drop support for java 7? theres plenty of old mods that do not work on java 8

If I may ask, which mods for example?

[Moresteck]

As for the PR, I agree using Java 8 for building would make the process easier, but I don't see why source and target versions should get bumped from 7 to 8. The source is still compatible with Java 7.
I'd suggest them to be left at 7 for both NewLaunch.jar and JavaCheck.jar.

[davidebeatrici]

The problem is that recent OpenJDK versions support a minimum version of 8.

[Moresteck]

The problem is that recent OpenJDK versions support a minimum version of 8.

But you'd be using JDK 8 to build MultiMC, no? At least that's what I would do.

[davidebeatrici]

Yes, OpenJDK 8 is still supported. If you all agree I will drop the source and target changes.