chore: add npm version constraint to renovate (#9496)

Renovate uses the latest npm version, however this is now causing a
issue where the latest npm version is diverging from the npm version
packaged with node v22 (version the CI uses) in terms of what the
`package-lock.json` should contain (!9477) (some `"peer": true` lines).

Constrain renovate to use npm v10 to avoid this mismatch issue.

Thank you @viceice for helping with this patch.

Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/9496
Reviewed-by: Michael Kriese <michael.kriese@gmx.de>
Co-authored-by: Gusted <postmaster@gusted.xyz>
Co-committed-by: Gusted <postmaster@gusted.xyz>

renovate.json

@@ -16,6 +16,9 @@
   "osvVulnerabilityAlerts": true,
   "automergeStrategy": "squash",
   "labels": ["dependency-upgrade", "test/not-needed"],
+  "constraints": {
+    "npm": "10"
+  },
   "packageRules": [
     {
       "description": "Require approval for python minor version",