e2e: Replace EOL CentOS 8 image in test templates

[bpickard22]

Problem

Several e2e test templates use quay.io/centos/centos:8 (or previously centos:8, being fixed in #1472). CentOS 8 reached end-of-life on December 31, 2021. The image is no longer receiving updates and poses several risks:

1. Image removal risk -- quay.io/centos/centos:8 could be removed from the registry at any time, causing all tests using it to fail with image pull errors.
2. Known CVEs -- The image contains unpatched vulnerabilities. CI environments with security scanning may flag or block it.
3. Stale base -- Built on glibc 2.28 and 2019-era kernel headers.

Affected files

• e2e/templates/default-route1.yml.j2 (2 references)
• e2e/templates/simple-macvlan1.yml.j2 (2 references)
• e2e/templates/simple-pod.yml.j2 (1 reference)

All of these pods just run /bin/sleep 10000 or simple network commands -- they don't depend on CentOS-specific functionality.

Suggested fix

Replace quay.io/centos/centos:8 with one of:

• quay.io/centos/centos:stream9 -- Drop-in replacement, actively maintained, lowest friction.
• docker.io/library/alpine:<pinned version> -- Smallest image, already used in cni-install.yml.j2, would unify all test pods on one base.

centos:stream9 is the simplest path since it requires no other changes (same package manager, same paths, same shell).

Related

• e2e: Use fully qualified image name for images #1472 -- Fixes the short-name issue for these same templates but retains the EOL image.