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)
- [Changelog](https://github.com/pydata/xarray/blob/main/HOW_TO_RELEASE.md)
- [Commits](pydata/xarray@v0.1...v2023.07.0)

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

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

.github/dependabot/constraints.txt

@@ -13,4 +13,4 @@ python-dateutil==2.8.2
 pytz==2023.3
 pyzmq==25.1.0
 six==1.16.0
-xarray<2023.6.0
+xarray<2023.8.0