[chore] update changelog for instana removal

Signed-off-by: Alex Boten <223565+codeboten@users.noreply.github.com>

.chloggen/codeboten_rm-instanaexporter.yaml

@@ -15,7 +15,7 @@ issues: [35367]
 # (Optional) One or more lines of additional information to render under the primary note.
 # These lines will be padded with 2 spaces and then inserted directly into the document.
 # Use pipe (|) for multiline entries.
-subtext:
+subtext: "Use the `otlp` exporter instead as explained in IBM's [documentation](https://www.ibm.com/docs/en/instana-observability/current?topic=opentelemetry-sending-data-instana-backend)."
 
 # If your change doesn't affect end users or the exported elements of any package,
 # you should instead start your pull request title with [chore] or use the "Skip Changelog" label.