Add OPNsense 21.7.1 release

Before submitting a pull request, please check the following.

When updating an existing appliance:

[x] The new version is on top.
[x] The filenames in the "images" section are unique, to avoid appliances / version overwriting each other.
[x] If you forked the repo, running check.py doesn't drop any errors for the updated file.

When creating a new appliance:
It's tested locally, i.e.
- [x] You dragged an instance into a project on your box, got it installed (if necessary), and did some basic network checks (ping, UI reachable, etc.).
- [x] GNS3 VM can run it without any tweaks.
- [x] The device is in the right category: router, switch, guest (hosts), firewall
- [x] You filled in as much info as possible (checks the schemas and other appliance files for some guidance).
[x] When adding a container: it builds on Docker Hub and can be pulled.
[x] The filenames in the "images" section are unique (to avoid appliances and/or versions overwriting each other).
[x] If you forked the repo, running check.py doesn't drop any errors for the new file.
[ ] Optional: a symbol has been created for the new appliance.

GNS3 / gns3-registry