switch to offical Antora plugin for Gradle

- lock version to latest release of Antora 3.1
- rename properties on extension block
- use Node.js version provided by plugin
- remove package.json file
- assign environment variables using environments property on extension block
- use single quotes where possible in build script
- use default setting for log format

docs/package.json

@@ -1,8 +0,0 @@
-{
-  "name": "spring-security-docs-antora",
-  "private": true,
-  "dependencies": {
-    "@antora/cli": "^3.1.0",
-    "@antora/site-generator-default": "^3.1.0"
-  }
-}