Bump org.apache.maven.plugins:maven-site-plugin from 3.12.1 to 3.21.0

Bumps [org.apache.maven.plugins:maven-site-plugin](https://github.com/apache/maven-site-plugin) from 3.12.1 to 3.21.0.
- [Release notes](https://github.com/apache/maven-site-plugin/releases)
- [Commits](apache/maven-site-plugin@maven-site-plugin-3.12.1...maven-site-plugin-3.21.0)

---
updated-dependencies:
- dependency-name: org.apache.maven.plugins:maven-site-plugin
  dependency-version: 3.21.0
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

spring-cloud-contract-tools/spring-cloud-contract-maven-plugin/pom.xml

@@ -37,7 +37,7 @@
 		<maven-archiver.version>3.6.3</maven-archiver.version>
 		<plexus-archiver.version>4.10.0</plexus-archiver.version>
 		<coveralls-maven-plugin.version>4.3.0</coveralls-maven-plugin.version>
-		<maven-site-plugin.version>3.12.1</maven-site-plugin.version>
+		<maven-site-plugin.version>3.21.0</maven-site-plugin.version>
 		<maven-plugin-testing-harness.version>3.3.0</maven-plugin-testing-harness.version>
 		<maven-shared-incremental.version>1.1</maven-shared-incremental.version>
 		<slf4j-api.version>2.0.7</slf4j-api.version>