art: rename kairos-agent and add sha256 signatures (#670)

* 🎨 Change kairos agent release name and sign sha256

This patch changes the release artifact to be named kairos-agent instead
of just kairos, so its clear what the archive is referring to.

Also adds cosign signatures to the iso sha256 files so users can confirm
they havent been tampered with

Signed-off-by: Itxaka <itxaka@spectrocloud.com>

* Add docs for signature and checksum verification for ISO

Signed-off-by: Itxaka <itxaka@spectrocloud.com>

* Update version used for signing docs

Signed-off-by: Itxaka <itxaka@spectrocloud.com>

---------

Signed-off-by: Itxaka <itxaka@spectrocloud.com>
Co-authored-by: Itxaka <itxaka@spectrocloud.com>

.goreleaser.yaml

@@ -1,4 +1,5 @@
 # Make sure to check the documentation at http://goreleaser.com
+project_name: kairos-agent
 builds:
   - ldflags:
       - -w -s
@@ -11,14 +12,14 @@ builds:
       - arm64
       - 386
     main: ./cmd/agent/
-    id: "kairos-agent"
-    binary: "kairos-agent"
+    binary: '{{ .ProjectName }}'
 source:
   enabled: true
   name_template: '{{ .ProjectName }}-{{ .Tag }}-source'
 archives:
   # Default template uses underscores instead of -
   - name_template: "{{ .ProjectName }}-{{ .Tag }}-{{ .Os }}-{{ .Arch }}{{ if .Arm }}v{{ .Arm }}{{ end }}"
+    # TODO: this is deprecated -> https://goreleaser.com/deprecations#archivesreplacements
     replacements:
       darwin: Darwin
       linux: Linux