linkage-checker-enforcer-rules incompatible with maven 3.9? ClassNotFound: org/eclipse/aether/connector/basic/BasicRepositoryConnectorFactory

I am trying to get the plugin working with a minimal pom.xml:

```

4.0.0

it.neckar
test-project
1.0-SNAPSHOT
jar

UTF-8

org.junit.jupiter
junit-jupiter
5.10.0
test

org.apache.maven.plugins
maven-enforcer-plugin
3.6.2

com.google.cloud.tools
linkage-checker-enforcer-rules
1.5.15

linkage-check

enforce

verify




true

```

Execution linkage-check of goal org.apache.maven.plugins:maven-enforcer-plugin:3.6.2:enforce failed: A required class was missing while executing org.apache.maven.plugins:maven-enforcer-plugin:3.6.2:enforce: org/eclipse/aether/connector/basic/BasicRepositoryConnectorFactory

It fails with
* mvn 3.9.*

Works up until
* mvn 3.8.9

從 issue 中的最小 pom.xml 開始，使用 Maven 3.9.* 和 3.8.9 重現 linkage-check 的執行。調查 linkage-checker-enforcer-rules 1.5.15 在 Maven 3.9 下無法載入 BasicRepositoryConnectorFactory 的原因；完成標準是在受影響的 Maven 版本上執行 check 時不再出現缺少類別的失敗。

由索引模型根據 Issue 內容生成。