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

gradle.properties

@@ -0,0 +1,2 @@
+group=org.springframework.security
+description=Spring Security Docs Site