build: remove inactive CE versions from being backport eligible (#23720)

As of Nomad 1.8.0 LTS we're no longer backporting changes to Nomad CE versions
1.7.x and 1.6.x. We never got around to removing the flags in the file the
backport assistant uses to control automated backports.

.release/versions.hcl

@@ -11,9 +11,9 @@ active_versions {
     lts       = true
   }
   version "1.7.x" {
-    ce_active = true
+    ce_active = false
   }
   version "1.6.x" {
-    ce_active = true
+    ce_active = false
   }
 }