The Cosign integration doesn't work with version 2.0.0+

[ThomasVitale]

Before version 2.0.0, Cosign would generate private keys with the following labels:

-----BEGIN ENCRYPTED COSIGN PRIVATE KEY-----
...
-----END ENCRYPTED COSIGN PRIVATE KEY-----

Since version 2.0.0, the labels used for the private keys have changed to the following:

-----BEGIN ENCRYPTED SIGSTORE PRIVATE KEY-----
...
-----END ENCRYPTED SIGSTORE PRIVATE KEY-----

The changes have been made in sigstore/cosign#2471.

That means the cosign generate-key-pair command (used in the tutorial) doesn't work anymore because kpack can't read the new format until the cosign dependency is upgraded in kpack.

I'm available to contribute a pull request, if needed.

[ThomasVitale]

I tried to give the change a try in #1197