Temurin is the Eclipse Foundation’s successor to AdoptOpenJDK
and is likely to continue to be the most widely-used community
distribution.
Unfortunately, Temurin only provides JDK 8, 11, and 16,
and since bitcoinj can’t yet be built with JDK 16, we’ll
drop the JDK 15 build without replacing it with JDK 16.
Use Github Actions built-in Gradle 5.6.2
Github Actions virtual environments all include
a pre-installed Gradle that is 5.6 or later, so
we can just build with it without using the wrapper.
