use ost 2.3.0 container as base image in Singularity recipe for 2.3.1

2.3.1 is a bugfix release and we didnt push a container for
2.3.1 as it is not affected

singularity/Singularity

 BootStrap: docker
-From: registry.scicore.unibas.ch/schwede/openstructure:2.3.1-focal
+From: registry.scicore.unibas.ch/schwede/openstructure:2.3.0-focal
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