[ci] Shut off 8.15 branch (#205788)

Author: jbudz

.buildkite/pipeline-resource-definitions/kibana-es-snapshots.yml

@@ -22,7 +22,7 @@ spec:
         SLACK_NOTIFICATIONS_CHANNEL: '#kibana-operations-alerts'
         ELASTIC_SLACK_NOTIFICATIONS_ENABLED: 'true'
       allow_rebuilds: true
-      branch_configuration: main 8.x 8.17 8.16 8.15 7.17
+      branch_configuration: main 8.x 8.17 8.16 7.17
       default_branch: main
       repository: elastic/kibana
       pipeline_file: .buildkite/pipelines/es_snapshots/build.yml
@@ -95,7 +95,7 @@ spec:
         SLACK_NOTIFICATIONS_CHANNEL: '#kibana-operations-alerts'
         ELASTIC_SLACK_NOTIFICATIONS_ENABLED: 'true'
       allow_rebuilds: true
-      branch_configuration: main 8.x 8.17 8.16 8.15 7.17
+      branch_configuration: main 8.x 8.17 8.16 7.17
       default_branch: main
       repository: elastic/kibana
       pipeline_file: .buildkite/pipelines/es_snapshots/promote.yml
@@ -144,7 +144,7 @@ spec:
         ELASTIC_SLACK_NOTIFICATIONS_ENABLED: 'true'
         REPORT_FAILED_TESTS_TO_GITHUB: 'true'
       allow_rebuilds: true
-      branch_configuration: main 8.x 8.17 8.16 8.15 7.17
+      branch_configuration: main 8.x 8.17 8.16 7.17
       default_branch: main
       repository: elastic/kibana
       pipeline_file: .buildkite/pipelines/es_snapshots/verify.yml

.buildkite/pipeline-resource-definitions/kibana-on-merge-unsupported-ftrs.yml

@@ -22,7 +22,7 @@ spec:
         SLACK_NOTIFICATIONS_CHANNEL: '#kibana-unsupported-ftrs-alerts'
         ELASTIC_SLACK_NOTIFICATIONS_ENABLED: 'true'
       allow_rebuilds: true
-      branch_configuration: main 8.x 8.17 8.16 8.15 7.17
+      branch_configuration: main 8.x 8.17 8.16 7.17
       default_branch: main
       repository: elastic/kibana
       pipeline_file: .buildkite/pipelines/on_merge_unsupported_ftrs.yml

.buildkite/pipeline-resource-definitions/kibana-on-merge.yml

@@ -25,7 +25,7 @@ spec:
         REPORT_FAILED_TESTS_TO_GITHUB: 'true'
         ELASTIC_SLACK_NOTIFICATIONS_ENABLED: 'true'
       allow_rebuilds: true
-      branch_configuration: main 8.x 8.17 8.16 8.15 7.17
+      branch_configuration: main 8.x 8.17 8.16 7.17
       default_branch: main
       repository: elastic/kibana
       pipeline_file: .buildkite/pipelines/on_merge.yml