Fix cold storage preview env

Existing cold storage preview env need some attentions:

The badge in https://github.com/nuxeo/nuxeo-coldstorage/blob/lts-2021/README.md redirects to https://preview-coldstorage-lts-2021.napps.dev.nuxeo.com/ but it should be https://preview-nuxeo-coldstorage-lts-2021.napps.dev.nuxeo.com/

Moreover, buckets used for preview are misconfigured, typically for 10.10, it is located in https://s3.console.aws.amazon.com/s3/buckets/napps-ci-coldstorage?prefix=nuxeo-coldstorage-10-10-39%2F&region=eu-west-1 and it is apparently missing the 's3:PutObject' permission for the policy 'napps-ci-coldstorage-policy' the https://us-east-1.console.aws.amazon.com/iam/home#/policies/arn:aws:iam::762822024843:policy/nxsec-stacks/napps-ci-coldstorage-policy

As a direct result, it seems the "Send to Cold Storage" action is not possible there.