📌(backend) unpin lxml and upgrade to 5.2.1

With the constraint on lxml<5, an error on the collectstatic occured about lxml
and xmlsec libxml2 versions mismatch. Unpin lxml to the latest release fix this
error. We give back renovate the permission to upgrade the package.

renovate.json

@@ -59,16 +59,6 @@
       ],
       "allowedVersions": "<5"
     },
-    {
-      "groupName": "allowed lxml versions",
-      "matchManagers": [
-        "setup-cfg"
-      ],
-      "matchPackageNames": [
-        "lxml"
-      ],
-      "allowedVersions": "<5"
-    },
     {
       "groupName": "allowed pytest versions",
       "matchManagers": [

src/backend/setup.cfg

@@ -54,7 +54,7 @@ install_requires =
     drf-spectacular==0.27.1
     gunicorn==21.2.0
     logging-ldp==0.0.7
-    lxml<5
+    lxml==5.2.1
     oauthlib==3.2.2
     Pillow==10.2.0
     psycopg[binary]==3.1.18