Update CentOS/RHEL images (gravitational#295)

These are already relatively current, but as long as we're updating
everything else we may as well catch these too.

assets/terraform/gce/os.tf

@@ -16,20 +16,20 @@ variable "oss" {
     "ubuntu:latest" = "ubuntu-os-cloud/ubuntu-2004-focal-v20210825"
 
     "redhat:7.8"    = "rhel-cloud/rhel-7-v20200910"
-    "redhat:7.9"    = "rhel-cloud/rhel-7-v20201112"
-    "redhat:7"      = "rhel-cloud/rhel-7-v20201112"
+    "redhat:7.9"    = "rhel-cloud/rhel-7-v20210817"
+    "redhat:7"      = "rhel-cloud/rhel-7-v20210817"
     "redhat:8.2"    = "rhel-cloud/rhel-8-v20200910"
     "redhat:8.3"    = "rhel-cloud/rhel-8-v20201112"
-    "redhat:8.4"    = "rhel-cloud/rhel-8-v20210701"
-    "redhat:8"      = "rhel-cloud/rhel-8-v20210701"
+    "redhat:8.4"    = "rhel-cloud/rhel-8-v20210817"
+    "redhat:8"      = "rhel-cloud/rhel-8-v20210817"
 
     "centos:7.8"    = "centos-cloud/centos-7-v20200910"
-    "centos:7.9"    = "centos-cloud/centos-7-v20201112"
-    "centos:7"      = "centos-cloud/centos-7-v20201112"
+    "centos:7.9"    = "centos-cloud/centos-7-v20210817"
+    "centos:7"      = "centos-cloud/centos-7-v20210817"
     "centos:8.2"    = "centos-cloud/centos-8-v20200910"
     "centos:8.3"    = "centos-cloud/centos-8-v20201112"
-    "centos:8.4"    = "centos-cloud/centos-8-v20210701"
-    "centos:8"      = "centos-cloud/centos-8-v20210701"
+    "centos:8.4"    = "centos-cloud/centos-8-v20210817"
+    "centos:8"      = "centos-cloud/centos-8-v20210817"
 
     "debian:8"      = "debian-cloud/debian-8-jessie-v20180611"
     "debian:9"      = "debian-cloud/debian-9-stretch-v20210817"