Default 3.6.1 is not installable

Hiya! I just tried using the version 7 release with the default, and it looks like the version set in the action.yaml (3.6.1) isn't allowed by the action:

```
Run eWaterCycle/setup-singularity@bebf1d6f54f9d2d159ac5c15385912dc39b23711
Setup singularity version spec 3.6.1
Version 3.6.1 was not found in the local cache
Binary build of version 3.6.1 is not available for downloading
Error: Unable to build Singularity binary for 3.6.1. Only support versions >=3.7.1
```
It might be time for a new release (soonish?) so the default (without params) works out of the box!