Add script to fetch OpenJDK 21 prebuilts am: f849b278a3 am: 6132427422

Original change: https://android-review.googlesource.com/c/platform/prebuilts/jdk/jdk21/+/2850965

Change-Id: Ia3a10a751a320e2d51e2ddc714656dc43789d6ea
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>