docs(semantic_text): add index version limitation (elastic#133374)

Author: mromaios

docs/reference/mapping/types/semantic-text.asciidoc

@@ -389,3 +389,4 @@ PUT test-index
 * `semantic_text` fields are not currently supported as elements of <<nested,nested fields>>.
 * `semantic_text` fields can't currently be set as part of <<dynamic-templates>>.
 * `semantic_text` fields are currently not supported with Cross-Cluster Search (CCS) or Cross-Cluster Replication (CCR).
+* `semantic_text` fields are not supported in indices created prior to 8.11.0.