Warnings created as render array don't work in D10

Problem/Motivation

An entity with a entity reference field that allows selection of unpublished content should generate a warning when saving. Currently it crashes with the following error:
strlen(): Argument #1 ($string) must be of type string, array given

This occurs in the Big Pipe core module where it expects a string or a markup class.

Steps to reproduce

1. Create an entity type with an entity reference field
2. Allow that field to reference unpublished content
3. Create an instance of that entity type referencing a unpublished entity and save it as published

Proposed resolution

Render the message render array before adding the warning

Remaining tasks

Create a patch

User interface changes

None

API changes

None

Data model changes

None