build(deps): update xarray requirement in /.github/dependabot

Updates the requirements on [xarray](https://github.com/pydata/xarray) to permit the latest version.
- [Release notes](https://github.com/pydata/xarray/releases)
- [Commits](pydata/xarray@v0.1...v2026.04.0)

---
updated-dependencies:
- dependency-name: xarray
  dependency-version: 2026.4.0
  dependency-type: direct:production
...

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

Author: dependabot[bot]

.github/dependabot/constraints.txt

@@ -13,4 +13,4 @@ python-dateutil==2.9.0.post0
 pytz==2026.1.post1
 pyzmq==27.1.0
 six==1.17.0
-xarray<2026.3.0
+xarray<2026.5.0