Fix nextjs image unoptimized

Bump the Next.js version so that build pass again after cloning.
In Next.js 12.3 the unoptimzed configuration became stable. Ref:
https://nextjs.org/blog/next-12-3#disable-image-optimization-stable

next.config.js

@@ -1,7 +1,5 @@
 module.exports = {
-    experimental: {
-        images: {
-            unoptimized: true
-        }
+    images: {
+        unoptimized: true
     }
-}
+}

package.json

@@ -124,7 +124,7 @@
     "autoprefixer": "^10.4.8",
     "electron": "^20.0.2",
     "electron-builder": "^23.3.3",
-    "next": "^12.2.5",
+    "next": "^12.3.1",
     "postcss": "^8.4.16",
     "prettier": "^2.7.1",
     "prettier-plugin-tailwindcss": "^0.1.13",