[ci] Use GCP prod image for Linux JDK matrix job (#15600) (#15602)

So far we've been using images from the -qa GCP image project throughput
the development of the Logstash Linux JDK matrix pipeline for quicker
iteration.

As we have scheduled weekly builds of those images that promote to
prod[^1] we can now switch to the prod version of the GCP images.

[^1]: https://buildkite.com/elastic/ci-vm-images/builds/2888

Relates https://github.com/elastic/ingest-dev/issues/1725

(cherry picked from commit e259e04e53)

Co-authored-by: Dimitrios Liappis <dimitrios.liappis@gmail.com>

.buildkite/scripts/jdk-matrix-tests/generate-steps.py

@@ -21,7 +21,7 @@ class JobRetValues:
 class GCPAgent:
     def __init__(self, image: str, machineType: str, diskSizeGb: int = 200, diskType: str = "pd-ssd") -> None:
         self.provider = "gcp"
-        self.imageProject = "elastic-images-qa"
+        self.imageProject = "elastic-images-prod"
         self.image = image
         self.machineType = machineType
         self.diskSizeGb = diskSizeGb