[docs] update getting-started to work with cosign 2.1

[NissesSenap]

Summary

Trying to follow the getting started guide is currently rather hard due to the cosign version used is rather old.
To make it easier I updated the documentation.

Release Note

NONE

Documentation

[NissesSenap]

I have tested all the commands with cosign version 2.1.0 I chose not to update the example output, personally I think it's close enough, and I think people will understand.

I also tried to run with cosign 2.2.0, but then I got the following error

Handling connection for 8080
Error: signing registry.local:5001/sigstore/demo@sha256:95bb686784c1d0deafc656486ae562e0420ab2b9a2f4c307a6571d401ce00e3e: [POST /api/v1/log/entries][422] createLogEntry default  &{Code:422 Message:invalid kind value: "dsse"}
main.go:74: error during command execution: signing registry.local:5001/sigstore/demo@sha256:95bb686784c1d0deafc656486ae562e0420ab2b9a2f4c307a6571d401ce00e3e: [POST /api/v1/log/entries][422] createLogEntry default  &{Code:422 Message:invalid kind value: "dsse"}

I'm 90% sure this is due to sigstore/cosign#3113.

I think this PR is better than the current state, but ideally it should work with 2.2.0.
Furthermore, I feel that I'm a bit out of depth trying to debug why this happens, but looking at the PR it seems to be a change in the way of working.
If anyone with more knowledge about the dsse change, please share.

[NissesSenap]

Thanks for the fix @bobcallaway , I was at PTO so that why I was so slow to update.
Is there anything else needed for the PR you feel?