Use headers of 18.20.4 for prebuilds

Previously we used 8.14.0 which is really old. We can't just omit
the `--target` argument and use the latest version, because Node.js
23 dropped support of 32-bit Windows.

The choice for 18.20.4 specifically is not really important. I just
picked LTS and then the latest version within that range. The intent
is 18.x.

Category: change

package.json

@@ -14,17 +14,17 @@
     "test-prebuild": "cross-env PREBUILDS_ONLY=1 npm t",
     "coverage": "nyc report -r lcovonly",
     "rebuild": "npm run install --build-from-source",
-    "prebuild": "prebuildify -t 8.14.0 --napi --strip",
+    "prebuild": "prebuildify -t 18.20.4 --napi --strip",
     "download-prebuilds": "prebuildify-ci download",
     "hallmark": "hallmark --fix",
     "dependency-check": "dependency-check --no-dev -i napi-macros . test/*.js",
     "prepublishOnly": "npm run dependency-check",
-    "prebuild-linux-arm": "prebuildify-cross -i linux-armv6 -i linux-armv7 -i linux-arm64-lts -t 8.14.0 --napi --strip",
-    "prebuild-android-arm": "prebuildify-cross -i android-armv7 -i android-arm64 -t 8.14.0 --napi --strip",
-    "prebuild-linux-x64": "prebuildify-cross -i centos7-devtoolset7 -i alpine -t 8.14.0 --napi --strip",
-    "prebuild-darwin-x64+arm64": "prebuildify -t 8.14.0 --napi --strip --arch x64+arm64",
-    "prebuild-win32-x86": "prebuildify -t 8.14.0 --napi --strip",
-    "prebuild-win32-x64": "prebuildify -t 8.14.0 --napi --strip"
+    "prebuild-linux-arm": "prebuildify-cross -i linux-armv6 -i linux-armv7 -i linux-arm64-lts -t 18.20.4 --napi --strip",
+    "prebuild-android-arm": "prebuildify-cross -i android-armv7 -i android-arm64 -t 18.20.4 --napi --strip",
+    "prebuild-linux-x64": "prebuildify-cross -i centos7-devtoolset7 -i alpine -t 18.20.4 --napi --strip",
+    "prebuild-darwin-x64+arm64": "prebuildify -t 18.20.4 --napi --strip --arch x64+arm64",
+    "prebuild-win32-x86": "prebuildify -t 18.20.4 --napi --strip",
+    "prebuild-win32-x64": "prebuildify -t 18.20.4 --napi --strip"
   },
   "dependencies": {
     "abstract-level": "^2.0.0",