Add a note for the moved Elasticsearch auto configuration and RestClientCustomizer in the release notes

[filiphr]

We were moving to Spring Boot 2.3 and noticed that the ES auto configuration classes were moved as part of fec386b. The RestClientBuilderCustomizer also got moved. Can this be mentioned in the release notes?

The move of the RestClientBuilderCustomizer is also a breaking change.

[philwebb]

Sorry about that, we'll update the docs. I've also opened #21572 to see if we can't reintroduce it in a deprecated form.

[filiphr]

Thanks for understanding

[snicoll]

I've added a note in the deprecation section. The original customizer has been restored in a deprecated fashion in #21572