[UPDATE] Update xapian-haystack requirement

Updates the requirements on [xapian-haystack](https://github.com/notanumber/xapian-haystack) to permit the latest version.
- [Changelog](https://github.com/notanumber/xapian-haystack/blob/master/CHANGELOG.rst)
- [Commits](https://github.com/notanumber/xapian-haystack/compare/3.1.0...4.0.0)

---
updated-dependencies:
- dependency-name: xapian-haystack
  dependency-version: 4.0.0
  dependency-type: direct:production
...

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

pyproject.toml

@@ -30,7 +30,7 @@ dependencies = [
     "phonenumbers>=9.0.25,<10.0.0",
     "reportlab>=4.4.10,<5.0.0",
     "django-haystack<4.0.0,>=3.3.0",
-    "xapian-haystack<4.0.0,>=3.1.0",
+    "xapian-haystack>=3.1.0,<5.0.0",
     "libsass<1.0.0,>=0.23.0",
     "django-ordered-model<4.0.0,>=3.7.4",
     "django-simple-captcha<1.0.0,>=0.6.3",