Update nextstrain-cli's dependency on s3fs

Adds the same minimum version bound used by our Python package on PyPI.
This lower bound is not strictly necessary for functionality, but it
ensures that this Conda package ends up with an s3fs version that uses
aiobotocore instead of much older s3fs versions that don't.  The
increased parity between the Python and Conda packages is useful to
avoid compatibility bugs only observed in one packaging or the other.

Related-to: <nextstrain/cli#261>

recipes/nextstrain-cli/meta.yaml

@@ -37,12 +37,12 @@ requirements:
     - wrapt
     # nextstrain-cli requires boto3 but does not directly declare this
     # dependency, to avoid issues with pip dependency resolution with s3fs and
-    # botocore. We can declare the boto3 dependency and unpinned s3fs here,
-    # since Conda can resolve these dependency issues.
+    # botocore. We can declare the boto3 dependency and s3fs here, since Conda
+    # can resolve these dependency issues.
     # See: https://github.com/nextstrain/cli/blob/77c97bfecaf3dda84a8c20c343afc70cc6659f53/setup.py#L91-L98
     - fsspec
     - boto3
-    - s3fs
+    - s3fs >=2021.04.0
 
 test:
   requires: