[CI] Increase onechat bundle limit (#235201)

## Summary
It seems slow increments to the bundle size at `onechat` grew over the
limits. This can be by having 2 PRs checked separately below limits, but
when both in main, they overgrew the previous limits.

This PR sets limits to 15709

This was the straw that broke the camel's back:
#235157
(https://buildkite.com/elastic/kibana-on-merge/builds/77176)

Author: delanni

packages/kbn-optimizer/limits.yml

@@ -115,7 +115,7 @@ pageLoadAssetSize:
   observabilityLogsExplorer: 4918
   observabilityOnboarding: 12872
   observabilityShared: 82382
-  onechat: 14281
+  onechat: 15709
   osquery: 47588
   painlessLab: 6299
   presentationPanel: 11484