Pin java8-multiarch at Forge compatible version

[itzg]

Enhancement Type

Improve an existing feature

Describe the enhancement

As reported here MultiMC/Launcher#4470 (comment) , Forge doesn't work with Java 8 update 321 or newer.

Should pin the java8-multiarch variant to 8u312-b07-jre

[shotah]

@itzg this can be closed.