🧪 Switch CentOS 7 yum repo mirrors to the vault (#83523)

The distro itself reached EoL on June 30, 2024, but these repository
archives live on and should remain usable for testing.

Refs:
* https://www.redhat.com/en/topics/linux/centos-linux-eol
* https://github.com/ansible/pylibssh/commit/01d3442f
* https://gist.github.com/hkneptune/4961532c2df691db79a1c6b72c251b25
* https://vault.centos.org/7.9.2009/

test/lib/ansible_test/_util/target/setup/bootstrap.sh

@@ -430,6 +430,15 @@ bootstrap_docker()
 {
     # Required for newer mysql-server packages to install/upgrade on Ubuntu 16.04.
     rm -f /usr/sbin/policy-rc.d
+
+    # CentOS 7 is EoL and its official repos are down; we need to the archived ones.
+    if grep -q '^CENTOS_MANTISBT_PROJECT="CentOS-7"$' /etc/os-release
+    then
+        sed -i \
+            -e 's/mirrorlist/#mirrorlist/g' \
+            -e 's|#baseurl=http://mirror.centos.org|baseurl=http://vault.centos.org|g' \
+            /etc/yum.repos.d/CentOS-*
+    fi
 }
 
 bootstrap_remote()