gradlePlugin { ... } is not part of the com.gradle.plugin-publish plugin, but part of the java-gradle-plugin which is built-in into Gradle and thus depends on the version of Gradle not of the plugin publish plugin. In Gradle 6.8.2 there is no website property in the extension of the java-gradle-plugin, that was only added in 7.6.
That’s btw. also what the docs at Gradle - How do I publish my plugin? say in the Examples section:
Plugin Publish plugin v1.0.0 simplifies the configuration of plugin publishing by merging the
pluginBundleblock into thegradlePluginone.
These simplifications however require backing support in the Gradle API, so are only available when using the Plugin Publish Plugin with Gradle v7.6 or later.
So I guess with 6.8.2 you just continue configuring the website and maybe other things in the pluginBundle block.