gerrit-main: Bump to Java 1.8
gerrit-main: Bump to Java 1.8

This is a preparation change to activate Error Prone for Buck driven
build. This fix is needed, otherwise the compilation is failing with:

  Source option 1.2 is no longer supported. Use 1.6 or later.
  Target option 1.2 is no longer supported. Use 1.6 or later.

Change-Id: I1ef45eee87dec788522873cabe4612bf7ac2ef5b

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