Migrate jobs from CentOS 8 to CentOS 8 Stream

CentOS 8 is being removed from CI because of its EOL[1].

[1] http://lists.openstack.org/pipermail/openstack-discuss/2022-January/026621.html

Change-Id: I55574f92d4b3934a4b4f14bf3dbd575c740d56b2

.zuul.yaml

@@ -17,7 +17,7 @@
 
 - job:
     name: validations-libs-reqcheck
-    nodeset: centos-8
+    nodeset: centos-8-stream
     parent: base
     run: playbooks/reqcheck.yaml
     timeout: 1600
@@ -29,7 +29,7 @@
 
 - job:
     name: validations-libs-coverchange
-    nodeset: centos-8
+    nodeset: centos-8-stream
     parent: base
     run: playbooks/coverchange.yaml
     timeout: 1600