fix: Use the default credential strategy in S3Client construction (#7061)

By omitting this option, we fall back to the hierarchy used by S3Client by
default.  When defined, the provider chain will use the values of
AWS_ACCESS_KEY_ID, AWS_SECRET_ACCESS_KEY (and AWS_SESSION_TOKEN); in their
absence, the provider chain can retrieve credentials from a range of other
sources, including e.g. ECS credentials.

Although there are no longer any application reads from `env.AWS_ACCESS_KEY_ID`
and `env.AWS_SECRET_ACCESS_KEY`, they continue to serve a useful documentary
role.

server/storage/files/S3Storage.ts

@@ -28,10 +28,6 @@ export default class S3Storage extends BaseStorage {
     this.client = new S3Client({
       bucketEndpoint: env.AWS_S3_ACCELERATE_URL ? true : false,
       forcePathStyle: env.AWS_S3_FORCE_PATH_STYLE,
-      credentials: {
-        accessKeyId: env.AWS_ACCESS_KEY_ID || "",
-        secretAccessKey: env.AWS_SECRET_ACCESS_KEY || "",
-      },
       region: env.AWS_REGION,
       endpoint: this.getEndpoint(),
     });