Zuul gerrit-base project: Downgrade java_version to 8
Zuul gerrit-base project: Downgrade java_version to 8

Upstream ensure-java role is changing the default to Java 11: [1].
Given that on 3.4 branch we are guaranteeing Java 8 compatibility,
downgrade the java version to 8.

Note that this change needs to be reverted when merged up to master.

[1] https://review.opendev.org/846909

Release-Notes: skip
Change-Id: I1b9826ec1796f965a1f78f6179fbe9945420bed2

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