Decimal fields rendered as number instead of string in OpenAPI schema

[clintonb]

Checklist

• I have verified that that issue exists against the master branch of Django REST framework.
• I have searched for similar issues in both open and closed tickets and cannot find a duplicate.
• This is not a usage question. (Those should be directed to the discussion group instead.)
• This cannot be dealt with as a third party library. (We prefer new functionality to be in the form of third party libraries where possible.)
• I have reduced the issue to the simplest possible case.
• I have included a failing test as a pull request. (If you are unable to do so we can still accept the issue.)

Steps to reproduce

1. Create a serializer with a decimal field.
2. Use the serializer in a ViewSet.
3. Render the OpenAPI schema.

Expected behavior

The decimal field should have a type of string.

Actual behavior

The decimal field has a type of number.

This issue was introduced by #6674.

[clintonb]

Additional context: Besides the schema being wrong—the API returns decimals as strings—JS/TS clients need to represent decimals as strings to avoid losing precision.