build: revert change to active versions

In #23720 we removed the 1.7.x and 1.6.x release branches from the "active"
versions for backports. But we forgot that we needed these backports to remain
active for documentation backports because of versioned docs.

.release/versions.hcl

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