Bump pystac from 1.6.1 to 1.8.4 in /rastervision_core

Bumps [pystac](https://github.com/stac-utils/pystac) from 1.6.1 to 1.8.4.
- [Release notes](https://github.com/stac-utils/pystac/releases)
- [Changelog](https://github.com/stac-utils/pystac/blob/main/CHANGELOG.md)
- [Commits](stac-utils/pystac@v1.6.1...v1.8.4)

---
updated-dependencies:
- dependency-name: pystac
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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

rastervision_core/requirements.txt

@@ -5,7 +5,7 @@ numpy==1.25.0
 pillow==10.0.1
 pyproj==3.4.0
 rasterio==1.3.7
-pystac==1.6.1
+pystac==1.8.4
 scikit-learn==1.2.2
 scipy==1.10.1
 opencv-python-headless==4.6.0.66