fix: add lts flag for 1.21 in versions.hcl (#22372)

Author: sreeram77

.release/versions.hcl

@@ -8,6 +8,7 @@ schema = 1
 active_versions {
   version "1.21" {
     ce_active = true
+    lts       = true
   }
   version "1.20" {}
   version "1.19" {}