Stop testing against JDK13 in the CI build

Description

JDK14 is GA, JDK13 is not an LTS, and we're testing plenty of JDKs already (8, 11, 13, 14, 15). Let's at least drop 13.

Environment

None

Assignee

Yoann Rodière

Reporter

Yoann Rodière

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Priority

Major
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