Updates release notes for 8.14.x to call for an update. (#16675)

Updates release notes for `8.14.x` to call for an update to a subsequent minor for fix performance regression in JSON decoding.

Co-authored-by: João Duarte <[email protected]>

docs/static/releasenotes.asciidoc

@@ -67,6 +67,14 @@ This section summarizes the changes in the following releases:
 [[logstash-8-14-3]]
 === Logstash 8.14.3 Release Notes
 
+[[known-issues-8-14-3]]
+==== Known issue
+
+**{ls} performance regression in JSON encoding**
+{ls} `8.14.1` fixed a bug in the JSON encoding of strings containing non-unicode data https://github.com/elastic/logstash/issues/15833[#15833].
+The fix introduced a performance regression that has since been solved with https://github.com/elastic/logstash/pull/16313[#16313] and included in {ls} `8.15.0`.
+There is no workaround for this issue, please upgrade to {ls} 8.15.0 or later.
+
 [[notable-8.14.3]]
 ==== Enhancements and notable issues fixed