Trac ticket number

ticket-35453

Branch description

Per discussion in PR #19595, ManyToManyField is likely misidentified as concrete. The is fixed by removing the non-existent column name from the field. This also removes the now redundant (field.is_relation and field.many_to_many) check and updates the field error to more explicitly point out that fields must be concrete.

There are several failures in the schema and migrations tests.

Checklist

• This PR targets the main branch.
• The commit message is written in past tense, mentions the ticket number, and ends with a period.
• I have checked the "Has patch" ticket flag in the Trac system.
• I have added or updated relevant tests.
• I have added or updated relevant docs, including release notes if applicable.
• I have attached screenshots in both light and dark modes for any UI changes.