📌(backend) pin pylint to 3.x to resolve compatibility conflict

Restrict pylint version to 3.x in renovate configuration because
pylint-django 2.6.1 requires pylint<4, preventing automatic upgrades
to pylint 4.x that would create unresolvable dependency conflicts
until pylint-django releases compatible version.

src/backend/pyproject.toml

@@ -74,7 +74,7 @@ dev = [
     "ipython==9.7.0",
     "pyfakefs==5.10.2",
     "pylint-django==2.6.1",
-    "pylint==4.0.2",
+    "pylint<4.0.0",
     "pytest-cov==7.0.0",
     "pytest-django==4.11.1",
     "pytest==9.0.0",