[8.19] [Dashboards as Code] Change access to dashboard route endpoints to internal (elastic#223262) (elastic#224060)

# Backport

This will backport the following commits from `main` to `8.19`:
- [[Dashboards as Code] Change access to dashboard route endpoints to
internal (elastic#223262)](elastic#223262)

<!--- Backport version: 10.0.1 -->

### Questions ?
Please refer to the [Backport tool
documentation](https://github.com/sorenlouv/backport)

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BACKPORT-->

---------

Co-authored-by: kibanamachine <[email protected]>

Author: nickpeihl