Fixes https://github.com/payloadcms/payload/issues/10787
This is the first part of the fix for
https://github.com/payloadcms/payload/issues/10787.
The second part is https://github.com/payloadcms/payload/pull/10785
The `packages/plugin-form-builder/src/types.ts` file contains many more
types besides MessageField.message that are unnecessarily `unknown` or
`any`.
In this PR I'm only fixing that one because:
1. It's what's needed to fix the issue.
2. I want to avoid a breaking change (they should be improved in v4
though).
I don't think MessageField.message will cause a breaking change for
anyone, since it's based on a rich text field.
5603c1ce8d