FRR: Update to version 8.1.0

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.

Updated the packer files for alpine-linux to use alpine version 3.15.0, then generated an updated FRR image and appliance, now with FRR version 8.1.0.

@grossmj Please download the FRR image from https://github.com/b-ehlers/gns3-registry/releases/download/frr/frr-8.1.0.qcow2 and upload it to SourceForge https://sourceforge.net/projects/gns-3/files/Qemu%20Appliances/.

GNS3 / gns3-registry

FRR: Update to version 8.1.0 #650

[x] If you forked the repo, running check.py doesn't drop any errors for the updated file.