Bump scippnexus from 24.11.1 to 25.4.0 in /requirements

Bumps [scippnexus](https://github.com/scipp/scippnexus) from 24.11.1 to 25.4.0.
- [Release notes](https://github.com/scipp/scippnexus/releases)
- [Commits](scipp/scippnexus@24.11.1...25.04.0)

---
updated-dependencies:
- dependency-name: scippnexus
  dependency-type: direct:production
  update-type: version-update:semver-major
...

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

Author: dependabot[bot]

requirements/base.txt

@@ -67,7 +67,7 @@ scipp==25.3.0
     # via
     #   -r base.in
     #   scippnexus
-scippnexus==24.11.1
+scippnexus==25.4.0
     # via -r base.in
 scipy==1.15.2
     # via