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: If6c2c55aaa13f030a4894fc7bdf54970dfc03a28

metadata.json

@@ -40,14 +40,12 @@
         {
             "operatingsystem": "RedHat",
             "operatingsystemrelease": [
-                "8",
                 "9"
             ]
         },
         {
             "operatingsystem": "CentOS",
             "operatingsystemrelease": [
-                "8",
                 "9"
             ]
         },

releasenotes/notes/remove-centos-8-a6433d4b189d29ab.yaml

@@ -0,0 +1,4 @@
+---
+upgrade:
+  - |
+    CentOS 8 Stream is no longer supported by this module.