Fix Java version for release job

[beatngu13]

See:

https://github.com/junit-pioneer/junit-pioneer/runs/5961527754?check_suite_focus=true

(Pointed out by emccue via Discord.)

Proposed commit message:

Fix Java version for release job (#620)

#608 bumped to Java version used in our CI to a minimum of 11, except
for the `release` job.

PR: #620

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