Jenkinsfile: remove CentOS 7 (EOL 2024-06-30)

CentOS 7 [reaches EOL on June 30][1], so we won't be building packages
for containerd 1.7. containerd 1.6 packages are still available for the
remaining time for those that need it.

This patch removes it from CI in this repository (but could still be built
in our release pipeline).

[1]: https://blog.centos.org/2023/04/end-dates-are-coming-for-centos-stream-8-and-centos-linux-7/#centos-linux-7-end-of-life-june-30-2024

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

Jenkinsfile

@@ -7,7 +7,6 @@
 // When adding a distro here, also open a pull request in the release repository.
 def images = [
     [image: "docker.io/library/amazonlinux:2",          arches: ["aarch64"]],
-    [image: "docker.io/library/centos:7",               arches: ["amd64", "aarch64"]],
     [image: "quay.io/centos/centos:stream9",            arches: ["amd64", "aarch64"]],          // CentOS Stream 9 (EOL: 2027)
     [image: "docker.io/library/rockylinux:8",           arches: ["amd64", "aarch64"]],          // Rocky Linux 8 (EOL: 2029-05-31)
     [image: "docker.io/library/rockylinux:9",           arches: ["amd64", "aarch64"]],          // Rocky Linux 9 (EOL: 2032-05-31)