(build) update latest GA'd JDKs to 16.

[bmarwell]

Following this checklist to help us incorporate your contribution quickly and easily:

• Make sure there is a JIRA issue filed
  for the change (usually before you start working on it). Trivial changes like typos do not
  require a JIRA issue. Your pull request should address just this issue, without pulling in other changes.
• Each commit in the pull request should have a meaningful subject line and body.
• Format the pull request title like [SHIRO-XXX] - Fixes bug in SessionManager,
  where you replace SHIRO-XXX with the appropriate JIRA issue. Best practice
  is to use the JIRA issue title in the pull request title and in the first line of the commit message.
• Write a pull request description that is detailed enough to understand what the pull request does, how, and why.
• Run mvn clean install apache-rat:check to make sure basic checks pass. A more thorough check will be performed on your pull request automatically.
• If you have a group of commits related to the same change, please squash your commits into one and force push your branch using git rebase -i.

Trivial changes like typos do not require a JIRA issue (javadoc, comments...).
In this case, just format the pull request title like (DOC) - Add javadoc in SessionManager.

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you have to acknowledge this by using the following check-box.

• I hereby declare this contribution to be licenced under the Apache License Version 2.0, January 2004
• In any other case, please file an Apache Individual Contributor License Agreement.

[bmarwell]

Blocked by: https://issues.apache.org/jira/browse/INFRA-21874