The openjdk ebuild is written to depend on either openjdk-bin or openjdk with the same slot to build it. Since there is not a corresponding openjdk-bin ebuild, there is no way to build this version of the package for the first time.

[ebuild  N     ] dev-java/openjdk-21.0.1_p12:21::java-kit  USE="alsa cups gentoo-vm (-big-endian) -debug -doc -examples -headless-awt -javafx (-selinux) -source -systemtap" 110,394 KiB                                                           

Total: 1 package (1 new), Size of downloads: 110,394 KiB

 * Error: circular dependencies:

(dev-java/openjdk-21.0.1_p12:21/21::java-kit, ebuild scheduled for merge) depends on
 (dev-java/openjdk-21.0.1_p12:21/21::java-kit, ebuild scheduled for merge) (buildtime)

 * Note that circular dependencies can often be avoided by temporarily
 * disabling USE flags that trigger optional dependencies.