build(deps): update sigstore_protobuf_specs requirement from 0.3 to 0.4

Updates the requirements on [sigstore_protobuf_specs](https://github.com/sigstore/protobuf-specs) to permit the latest version.
- [Release notes](https://github.com/sigstore/protobuf-specs/releases)
- [Changelog](https://github.com/sigstore/protobuf-specs/blob/main/CHANGELOG.md)
- [Commits](sigstore/protobuf-specs@v0.3.0...v0.4.0)

---
updated-dependencies:
- dependency-name: sigstore_protobuf_specs
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <[email protected]>

Cargo.toml

@@ -138,7 +138,7 @@ serde_json = "1.0"
 serde_with = { version = "3.9", features = ["base64", "json"], optional = true }
 sha2 = { version = "0.10", features = ["oid"] }
 signature = { version = "2.2" }
-sigstore_protobuf_specs = { version = "0.3", optional = true }
+sigstore_protobuf_specs = { version = "0.4", optional = true }
 thiserror = "2.0"
 tokio = { version = "1", features = ["rt"] }
 tokio-util = { version = "0.7", features = ["io-util"] }