Update specutils requirement

Updates the requirements on [specutils](https://github.com/astropy/specutils) to permit the latest version.
- [Release notes](https://github.com/astropy/specutils/releases)
- [Changelog](https://github.com/astropy/specutils/blob/main/CHANGES.rst)
- [Commits](astropy/specutils@v1.9.0...v1.16.0)

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

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

requirements.txt

@@ -8,4 +8,4 @@ plotly>=5.14.0.a, <=5.22.0
 pyvo>=1.4.1, <=1.5.2
 regions>=0.7, <=0.8
 requests>=2.4.3
-specutils>=1.9.0.a, <=1.15.0
+specutils>=1.9.0.a, <=1.16.0