Umbrella: JDK 15 terminal deprecations

This is an umbrella bug for tracking items that have been deprecated for removal.

When an API or feature is deprecated for removal (or when its ordinary deprecation is upgraded to a terminal deprecation), an issue corresponding to the removal of that API or feature should also be filed. There should be separate issues for tracking removals of individual or closely related groups of APIs. They should be linked to this issue using the "blocks" / "is blocked by" relationship, so this issue cannot be closed until all the block items are either completed or are moved to an umbrella issue for a subsequent release.

Use "top-level" issues (enhancements or bugs) for tracking each removal. Do not use sub-tasks as removal issues, since each will need a release note, and those need to be sub-tasks. (It's not possible in JIRA to create a sub-task of a sub-task.)

A terminally deprecated API or feature should typically be removed no earlier than one year from the date of deprecation, which typically means the second feature release in the six-month cycle. If something isn't going to be removed from the current release, its Fix-Version should be set and it should be unlinked from this umbrella removal bug and linked to the umbrella removal bug for the next release. This helps ensure that the decision to remove a terminally deprecated feature is revisited regularly.