Remove 7.x-only operating systems from pull request checks for now

.ci/jobs.t/elastic+elasticsearch+pull-request+packaging-tests-unix.yml

@@ -32,19 +32,15 @@
           type: label-expression
           name: os
           values:
-            - centos-6-packaging
             - centos-7-packaging
             - centos-8-packaging
-            - debian-8-packaging
             - debian-9-packaging
             - debian-10-packaging
             - opensuse-15-1-packaging
-            - oraclelinux-6-packaging
             - oraclelinux-7-packaging
             - oraclelinux-8-packaging
             - sles-12-packaging
             - sles-15-packaging
-            - ubuntu-16.04-packaging
             - ubuntu-18.04-packaging
             - ubuntu-20.04-packaging
       - axis: