Concurrency project housekeeping

[KyleAure]

This PR does the following housekeeping for the Concurrency project:

• Updates the CI builds to use Java 21 GA (instead of EA)
• Updates issue templates to be valid YML
• Adds a release template that will make it easier to keep track of tasks needed to create a release
• Adds a workflow that will automatically update our issue templates with updated versions
• Updates the signature file generation task to automatically copy and rename the signature file into the TCK resources.
• Creates a workflow that will automatically open a PR to update the signature files when updates to API are merged.
• Removes the requirement for updating the signature test based on Jakarta + JDK version support (instead we can just assume if a signature file for a certain JDK version doesn't exist then it isn't supported by the Jakarta release)

[mswatosh]

Looks good, the automation additions will be nice. I left one comment for a niche situation. Should probably leave the PR open for a few days in case anyone else wants to review.