Remove support for CentOS 8 Stream

... because RDO will provide packages for only CentOS Stream 9 for Zed
release. This change removes RHEL 8 as well.

Depends-on: https://review.opendev.org/843503
Change-Id: I8c5b682b4ab977a2c6ed747229ae553be471481f

metadata.json

@@ -36,14 +36,12 @@
         {
             "operatingsystem": "RedHat",
             "operatingsystemrelease": [
-                "8",
                 "9"
             ]
         },
         {
             "operatingsystem": "CentOS",
             "operatingsystemrelease": [
-                "8",
                 "9"
             ]
         },

releasenotes/notes/remove-centos-8-4e6642216fdf98a9.yaml

@@ -0,0 +1,5 @@
+---
+upgrade:
+  - |
+    CentOS 8 Stream and Red Hat Enterprise Linux 8 are no longer supported by
+    this module.