Upgrade `STORAGES` setting to the new pattern

[humitos]

This is a follow up of the changes introduced in:

• Upload staticfiles to S3 #12220

We upgrade our settings to be like:

    @property
    def STORAGES(self):
        return {
            "staticfiles": {
                "BACKEND": self.RTD_STATICFILES_STORAGE,
                "OPTIONS": {
                    "bucket_acl": "public-read",
                    "default_acl": "public-read",
                    "querystring_auth": False,
                },
            },

            "build-media": {
                "BACKEND": self.RTD_BUILD_MEDIA_STORAGE,
                "OPTIONS": {
                    "bucket_name": self.S3_MEDIA_STORAGE_BUCKET,
                    "custom_domain": self.S3_MEDIA_STORAGE_OVERRIDE_HOSTNAME,
                    "bucket_acl": "public-read",
                    "default_acl": "public-read",
                    "querystring_auth": False,
                },
            },

            # ... other storage backends here like `build-tools` and more ...
        }

With this, we will be able to delete all our classes and mixins to deal with multiple storages and make the code cleaner while using the Django standards:

• https://github.com/readthedocs/readthedocs.org/blob/humitos%2Fstaticfiles/readthedocs/storage/__init__.py#L43-L46
• https://github.com/readthedocs/readthedocs.org/blob/humitos%2Fstaticfiles/readthedocs/storage/s3_storage.py#L26-L121