Remove end of life Java versions from our testing matrix (#106042)

These Java versions are EOL and no longer supported by Elasticsearch so
we can remove them from our CI testing. We only need to support LTS
versions >= 17 and the currently latest bundled JDK version.

.buildkite/pipelines/periodic.template.yml

@@ -69,9 +69,6 @@ steps:
             ES_RUNTIME_JAVA:
               - graalvm-ce17
               - openjdk17
-              - openjdk18
-              - openjdk19
-              - openjdk20
               - openjdk21
               - openjdk22
             GRADLE_TASK:

.buildkite/pipelines/periodic.yml

@@ -1250,9 +1250,6 @@ steps:
             ES_RUNTIME_JAVA:
               - graalvm-ce17
               - openjdk17
-              - openjdk18
-              - openjdk19
-              - openjdk20
               - openjdk21
               - openjdk22
             GRADLE_TASK: