[Chore]: Update ra-core to use ReactNode instead of ReactElement where applicable

[jonathanarnault]

Problem

A lot of RA components expect a ReactElement where a ReactNode could be used.

Solution

Replace ReactElement with ReactNode where applicable.

How To Test

N/A

Additional Checks

• The PR targets master for a bugfix or a documentation fix, or next for a feature
• The PR includes unit tests (if not possible, describe why)
• The PR includes one or several stories (if not possible, describe why)
• The documentation is up to date

Also, please make sure to read the contributing guidelines.

[djhi]

Although this is obviously the correct thing to do, I wonder if we should release this at least in a minor version. @slax57, @fzaninotto thoughts?

[fzaninotto]

Although this is obviously the correct thing to do, I wonder if we should release this at least in a minor version. @slax57, @fzaninotto thoughts?

I tend to agree we should put this in a minor version.