For passers-by;
This issue dates back to Update 2023-11-13, and is listed under the heading of Changes in JDK / JRE 21 packages may require manual intervention
The following (revised) procedure has been suggested:
sudo pacman -Sy jdk-openjdk
# Choose `Y` (Yes) to remove each conflicting package
sudo pacman -Syu
# Answer `Y` (Yes) to rename/replace all packages indicated.
Or, if using Pamac:
pamac install jdk-openjdk --no-upgrade; pamac update --no-aur