build(deps): bump securesystemslib in the dependencies group

Bumps the dependencies group with 1 update: [securesystemslib](https://github.com/secure-systems-lab/securesystemslib).


Updates `securesystemslib` from 1.2.0 to 1.3.0
- [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@v1.2.0...v1.3.0)

---
updated-dependencies:
- dependency-name: securesystemslib
  dependency-version: 1.3.0
  dependency-type: direct:production
  update-type: version-update:semver-minor
  dependency-group: dependencies
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

requirements/pinned.txt

@@ -10,7 +10,7 @@ cryptography==44.0.2
     # via securesystemslib
 pycparser==2.22
     # via cffi
-securesystemslib==1.2.0
+securesystemslib==1.3.0
     # via -r requirements/main.txt
 urllib3==2.4.0
     # via -r requirements/main.txt