Manually fix BWC versions for tests

These BWC versions are updated by release-tools during FF or release
finalization. However, this is done only for development branches
currently.
We need to update the process to also update this branch.
This time it is done manually.

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"