Upgrade to Java 21

[bgprudhomme]

No description provided.

[bgprudhomme]

This is confusing me quite a bit:

OptimizerTests.testOptimizeLogLikelihood is supposed to run for 5 * 2 * 2 = 20 different combinations of parameters
However on the java21_rethreader branch, the Travis build labeled "Branch" often hangs after printing "Optimizing finished" fewer than 20 times. (The build labeled "Pull Request" is passing though, and on the main_rethreader branch, the "Branch" build passed.)
In my latest commit (which fixed up the gradle versioning), the build from "Pull Request" is passing, but the build from "Branch" only had 19 printouts (1 away!) and then stopped (https://app.travis-ci.com/github/ponder-lab/CoreNLP/builds/271225406). In the build before that, the build from "Pull Request" passed but the build from "Branch" only printed 12 times and then stopped (https://app.travis-ci.com/github/ponder-lab/CoreNLP/builds/271222022).
On the fibers_rethreader branch, there is only one Travis build (the one labeled "Branch"), the only one of which stops after only 2 printouts (https://app.travis-ci.com/github/ponder-lab/CoreNLP/builds/271223615).

Locally, when I run mvn clean install a bunch of times, I get inconsistent results. Here they are (with number of printouts out of 20 shown for fails):
On java21_rethreader: Fail (0), Pass, Fail (1), Pass, Fail (1), Fail (5), Fail (7), Pass, Fail (2), Fail (2)
On fibers_rethreader: Pass, Fail (7), Pass, Pass, Fail (13), Fail (2), Pass, Pass, Pass, Fail (19)

Could this be a flaky test?

[khatchad]

We're not using Gradle in the CI, so no need to change the Gradle files.

[bgprudhomme]

We're not using Gradle in the CI, so no need to change the Gradle files.

Fixed.