Bump securesystemslib from 0.13.1 to 0.28.0

Bumps [securesystemslib](https://github.com/secure-systems-lab/securesystemslib) from 0.13.1 to 0.28.0.
- [Release notes](https://github.com/secure-systems-lab/securesystemslib/releases)
- [Changelog](https://github.com/secure-systems-lab/securesystemslib/blob/main/CHANGELOG.md)
- [Commits](secure-systems-lab/securesystemslib@v0.13.1...v0.28.0)

---
updated-dependencies:
- dependency-name: securesystemslib
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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

pyproject.toml

@@ -48,7 +48,7 @@ keywords = [
 # pyca/cryptography).  *Verification* of either signature type does NOT
 # require securesystemslib.
 # WARNING: DEPENDENCY ON SECURESYSTEMSLIB PINNED.
-gpgsigning = ["securesystemslib==0.13.1"]
+gpgsigning = ["securesystemslib==0.28.0"]
 
 [project.urls]
 repository = "https://github.com/conda/conda-content-trust"