Manually fix BWC versions for tests (elastic#136169)

These BWC versions are updated automatically during feature freeze and release finalization.
However, this is done only for development branches currently.
We need to update the automation to also update this branch. This time it is done manually.

This should not be merged back to main.

Author: jozala

.buildkite/pipelines/intake.yml

@@ -65,7 +65,7 @@ steps:
         timeout_in_minutes: 300
         matrix:
           setup:
-            BWC_VERSION: ["8.18.9", "8.19.6", "9.1.6", "9.2.0", "9.3.0"]
+            BWC_VERSION: ["9.1.6", "9.2.0", "9.3.0"]
         agents:
           provider: gcp
           image: family/elasticsearch-ubuntu-2404

.buildkite/pipelines/periodic-java-ea.yml

@@ -538,7 +538,7 @@ steps:
         timeout_in_minutes: 300
         matrix:
           setup:
-            BWC_VERSION: ["8.18.9", "8.19.6", "9.1.6", "9.2.0", "9.3.0"]
+            BWC_VERSION: ["9.1.6", "9.2.0", "9.3.0"]
         agents:
           provider: gcp
           image: family/elasticsearch-ubuntu-2404

.buildkite/pipelines/periodic.yml

@@ -525,7 +525,7 @@ steps:
           setup:
             ES_RUNTIME_JAVA:
               - openjdk21
-            BWC_VERSION: ["8.18.9", "8.19.6", "9.1.6", "9.2.0", "9.3.0"]
+            BWC_VERSION: ["9.1.6", "9.2.0", "9.3.0"]
         agents:
           provider: gcp
           image: family/elasticsearch-ubuntu-2404

.ci/snapshotBwcVersions

@@ -1,6 +1,4 @@
 BWC_VERSION:
-  - "8.18.9"
-  - "8.19.6"
   - "9.1.6"
   - "9.2.0"
   - "9.3.0"