dependabot maven(deps): bump maven-site-plugin from 3.20.0 to 3.21.0

Bumps maven-site-plugin from 3.20.0 to 3.21.0.

Commits

• 43f73ec [maven-release-plugin] prepare release maven-site-plugin-3.21.0
• a2880fb [MSITE-1024] Remove IT for MSITE-901
• 6171e7d [MSITE-1023] Upgrade plugins and components (in ITs)
• efd8a57 [MSITE-1022] Upgrade to Maven Reporting Exec 2.0.0
• f5c5fc9 Fix typos in history.apt and faq.fml
• 64c4035 [MSITE-1021] Bump doxiaSitetoolsVersion from 2.0.0-M19 to 2.0.0
• e26765c [MSITE-1020] Bump org.apache.maven.reporting:maven-reporting-api from 4.0.0-M...
• 354cf19 [MSITE-1019] Bump doxiaVersion from 2.0.0-M12 to 2.0.0
• 67b568a Use '@​project.' instead of '@​pom.' expression prefix
• fdfd807 Add version 3.20.0 to plugin history
• Additional commits viewable in compare view