build(deps): update synphot requirement

Updates the requirements on [synphot](https://github.com/spacetelescope/synphot_refactor) to permit the latest version.
- [Release notes](https://github.com/spacetelescope/synphot_refactor/releases)
- [Changelog](https://github.com/spacetelescope/synphot_refactor/blob/master/CHANGES.rst)
- [Commits](spacetelescope/synphot_refactor@1.4.0...1.5.0)

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

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

requirements.txt

@@ -6,5 +6,5 @@ photutils>=1.13.0,<2.1.0
 poppy>=1.0.0
 pysiaf>=0.23.3,<=0.24.1
 scipy>=1.14.0,<1.15.0
-synphot>=1.4.0,<1.5.0
+synphot>=1.4.0,<1.6.0
 astroquery>=0.4.7,<0.5.0