HEMCO submod update: Merge PR #280 (Fix stale.yml file)

This commit informs the GCClassic superproject about the following
commits that were pushed to the GitHub geoschem/geos-chem repository:

ef3bde148 Merge PR #280 (Now use comma-separated list in stale.yml )

This PR fixes an improperly-formatted stale.yml file, which caused
the "Mark stale issues" GitHub action to fail.

Signed-off-by: Bob Yantosca <[email protected]>