Upgrade cosign to version v2.6.1 (#720)

Updated cosign installation to version v2.6.1.

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

## Motivation and Context
<!-- Why is this change needed? What problem does it solve? -->
The following pins cosign to v2.6.1. 

For more context:
- https://github.com/goreleaser/goreleaser/issues/6195
- https://github.com/modelcontextprotocol/registry/pull/706
## How Has This Been Tested?
<!-- 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
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-->

.github/workflows/release.yml

@@ -26,6 +26,8 @@ jobs:
 
       - name: Install cosign
         uses: sigstore/cosign-installer@faadad0cce49287aee09b3a48701e75088a2c6ad
+        with:
+          cosign-release: "v2.6.1"
 
       - name: Install Syft
         uses: anchore/sbom-action/download-syft@v0.20.9