Remove Ubuntu 18.04 from CI jobs (#15855) (#15859)

Relates https://github.com/elastic/ingest-dev/issues/2849

(cherry picked from commit 3f5b44a1ad)

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

.buildkite/linux_jdk_matrix_pipeline.yml

@@ -19,8 +19,6 @@ steps:
             value: "ubuntu-2204"
           - label: "Ubuntu 20.04"
             value: "ubuntu-2004"
-          - label: "Ubuntu 18.04"
-            value: "ubuntu-1804"
           - label: "Debian 11"
             value: "debian-11"
           - label: "Debian 10"

.buildkite/scripts/common/vm-images.json

@@ -1,7 +1,7 @@
 {
     "#comment": "This file lists all custom vm images. We use it to make decisions about randomized CI jobs.",
     "linux": {
-        "ubuntu": ["ubuntu-2204", "ubuntu-2004", "ubuntu-1804"],
+        "ubuntu": ["ubuntu-2204", "ubuntu-2004"],
         "debian": ["debian-11", "debian-10"],
         "rhel": ["rhel-9", "rhel-8"],
         "oraclelinux": ["oraclelinux-8", "oraclelinux-7"],