switch to official Antora Plugin for Gradle

- lock version to latest release of Antora 3.1
- declare Node.js dependencies in build.gradle
- use Node.js version provided by plugin
- assign environment variables using environments property on extension block
- move group and description project properties to gradle.properties
- remove unneeded .npmrc
- upgrade to Gradle 7.5.1
- pass -q when running gradlew from Collector

package.json

@@ -1,15 +0,0 @@
-{
-  "name": "spring-security-docs-site",
-  "description": "Spring Security Docs",
-  "private": true,
-  "dependencies": {
-    "@antora/cli": "^3.1.0",
-    "@antora/site-generator-default": "^3.1.0",
-    "@antora/collector-extension": "1.0.0-alpha.2"
-  },
-  "overrides": {
-    "vinyl-fs": {
-      "glob-stream": "~7.0"
-    }
-  }
-}