Changes observability plugin codeowner (#70439)

The observability plugin is now code-owned by a new observability-ui GH team to avoid pinging 4 separate teams for mandatory reviews when that plugin changes.

.github/CODEOWNERS

@@ -84,7 +84,7 @@
 /x-pack/plugins/infra/ @elastic/logs-metrics-ui
 /x-pack/plugins/ingest_manager/ @elastic/ingest-management
 /x-pack/legacy/plugins/ingest_manager/ @elastic/ingest-management
-/x-pack/plugins/observability/ @elastic/logs-metrics-ui @elastic/apm-ui @elastic/uptime @elastic/ingest-management
+/x-pack/plugins/observability/ @elastic/observability-ui
 /x-pack/legacy/plugins/monitoring/ @elastic/stack-monitoring-ui
 /x-pack/plugins/monitoring/ @elastic/stack-monitoring-ui
 /x-pack/plugins/uptime @elastic/uptime