[ECS release notes] Remove from reference

[KOTungseth]

Type of issue

Inaccurate

What documentation page is affected

https://www.elastic.co/docs/reference/ecs/ecs-release-notes

What happened?

The ECS release notes live here: https://www.elastic.co/docs/release-notes/ecs

This page should be removed from the Reference section.

Additional info

No response