linkage-checker-enforcer-rules compatibility documentation

According to https://maven.apache.org/resolver/maven-3.8.x.html
the maven-resolver-api versions 1.7.0 and up are no longer compatible with Maven 3.8.X

This was changed here with only a patch version bump. There's also no mention of this in the documentation.
I would think com.google.cloud.tools:linkage-checker-enforcer-rules:1.5.9 is the last supported version for Maven 3.8.X

Start by checking the repository's existing documentation for linkage-checker-enforcer-rules and compare its Maven compatibility claims with the Maven 3.8.x compatibility page linked in the issue. Document that maven-resolver-api 1.7.0 and later are incompatible with Maven 3.8.x, and identify 1.5.9 as the last supported linkage-checker-enforcer-rules version if the repository confirms it.

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