Update scikit-learn requirement from >=1.5.0 to >=1.8.0 in /backend

Updates the requirements on [scikit-learn](https://github.com/scikit-learn/scikit-learn) to permit the latest version.
- [Release notes](https://github.com/scikit-learn/scikit-learn/releases)
- [Commits](scikit-learn/scikit-learn@1.5.0...1.8.0)

---
updated-dependencies:
- dependency-name: scikit-learn
  dependency-version: 1.8.0
  dependency-type: direct:production
...

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

Author: dependabot[bot]

backend/pyproject.toml

@@ -105,7 +105,7 @@ dependencies = [
     "opencv-python-headless>=4.10.0.84",
     "psycopg[c]==3.2.12",
     "sentence-transformers>=3.0.0",
-    "scikit-learn>=1.5.0",
+    "scikit-learn>=1.8.0",
     "bertopic>=0.16.0",
     "nltk>=3.8.0",
 ]