Bump specutils from 1.14.0 to 1.15.0

Bumps [specutils](https://github.com/astropy/specutils) from 1.14.0 to 1.15.0.
- [Release notes](https://github.com/astropy/specutils/releases)
- [Changelog](https://github.com/astropy/specutils/blob/main/CHANGES.rst)
- [Commits](astropy/specutils@v1.14.0...v1.15.0)

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

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