Set minimum required Node version to 10.13

This was computed based on our dependencies using `installed-check`.
Node 10 is supported until April 2021.

Signed-off-by: David Mehren <git@herrmehren.de>
Co-authored-by: Yannick Bungers <git@innay.de>

package.json

@@ -143,7 +143,7 @@
     "**/request": "^2.88.0"
   },
   "engines": {
-    "node": ">=8.x"
+    "node": ">=10.13"
   },
   "bugs": "https://github.com/codimd/server/issues",
   "keywords": [