Update Goreleaser and Cosign (#839)

<!-- Provide a brief summary of your changes -->

## Motivation and Context
<!-- Why is this change needed? What problem does it solve? -->
The following PR bumps cosign and updates Goreleaser to reflect that.

Follows
https://goreleaser.com/blog/cosign-v3/#updating-your-goreleaser-configuration

<!-- Have you tested this in a real application? Which scenarios were
tested? -->

## Breaking Changes
<!-- Will users need to update their code or configurations? -->

## Types of changes
<!-- What types of changes does your code introduce? Put an `x` in all
the boxes that apply: -->
- [ ] Bug fix (non-breaking change which fixes an issue)
- [ ] New feature (non-breaking change which adds functionality)
- [ ] Breaking change (fix or feature that would cause existing
functionality to change)
- [ ] Documentation update

## Checklist
<!-- Go over all the following points, and put an `x` in all the boxes
that apply. -->
- [ ] I have read the [MCP
Documentation](https://modelcontextprotocol.io)
- [ ] My code follows the repository's style guidelines
- [ ] New and existing tests pass locally
- [ ] I have added appropriate error handling
- [ ] I have added or updated documentation as needed

## Additional context
<!-- Add any other context, implementation notes, or design decisions
-->

Signed-off-by: Radoslav Dimitrov <radoslav@stacklok.com>

.github/workflows/release.yml

@@ -25,9 +25,7 @@ jobs:
           cache: true
 
       - name: Install cosign
-        uses: sigstore/cosign-installer@faadad0cce49287aee09b3a48701e75088a2c6ad
-        with:
-          cosign-release: "v2.6.1"
+        uses: sigstore/cosign-installer@faadad0cce49287aee09b3a48701e75088a2c6ad # v4.0.0
 
       - name: Install Syft
         uses: anchore/sbom-action/download-syft@v0.20.10

.goreleaser.yaml

@@ -95,26 +95,24 @@ signs:
     cmd: cosign
     args:
       - "sign-blob"
-      - "--output-signature=${signature}"
-      - "--output-certificate=${certificate}"
+      - "--bundle=${signature}" # cosign v3+: bundles signature and certificate together
       - "${artifact}"
       - "--yes" # needed on cosign 2.0.0+
     artifacts: archive
     output: true
-    certificate: '{{ trimsuffix (trimsuffix .Env.artifact ".zip") ".tar.gz" }}.pem'
+    signature: "${artifact}.sigstore.json"
 
   # Also sign checksums file for additional verification
   - id: checksums
     cmd: cosign
     args:
       - "sign-blob"
-      - "--output-signature=${signature}"
-      - "--output-certificate=${certificate}"
+      - "--bundle=${signature}" # cosign v3+: bundles signature and certificate together
       - "${artifact}"
       - "--yes"
     artifacts: checksum
     output: true
-    certificate: '{{ trimsuffix .Env.artifact ".txt" }}.pem'
+    signature: "${artifact}.sigstore.json"
 
 # This section defines the release format.
 archives: