validate-packages must only validate artifacts of type "content-package"

Otherwise you might run into issues like
 
[ERROR] Failed to execute goal org.apache.jackrabbit:filevault-package-maven-plugin:1.3.2:validate-package (validate-package-cloud) on project acs-aem-commons-all: Could not validate package '/Users/davidg/Code/acs-aem-commons/target/checkout/all/target/acs-aem-commons-all-6.3.0-cloud.zip.asc': zip END header not found -> [Help 1]

(https://github.com/Adobe-Consulting-Services/acs-aem-commons/pull/3149#issuecomment-1779897403).

Currently only classifier is checked but not type/extension.