Remove CentOS 7 from CI (#16293)

CentOS 7 is EOL since June 30 2024[^1]. All repositories and mirrors are
now unreachable.

This commit removes CentOS 7 from CI jobs using it.

Relates elastic/ingest-dev#3520

[^1]: https://www.redhat.com/en/topics/linux/centos-linux-eol

.buildkite/linux_jdk_matrix_pipeline.yml

@@ -32,8 +32,6 @@ steps:
             value: "rhel-9"
           - label: "RHEL 8"
             value: "rhel-8"
-          - label: "CentOS 7"
-            value: "centos-7"
           - label: "Oracle Linux 8"
             value: "oraclelinux-8"
           - label: "Oracle Linux 7"