Correct database collation recommendation
Good question. There's some discussion here about utf8mb4_unicode_520_ci or utf8mb4_0900_ai_ci, but there's maybe some issues with that since at the moment in the associated changes it seems to be staying at utf8_unicode_ci. But it might change.

But the warning might be due to your global settings. In order to support this you need barracuda format available and some innodb settings. Most likely this one is not the default in your install: innodb_large_prefix=true.

Also need:

innodb_file_format=barracuda

innodb_file_per_table=true