Rename ArtifactoryPluginConvention to ArtifactoryPluginExtension

[Goooler]

Better alignment with the migration of convention usages.

Follow up #72.

• All tests passed. If this feature is not already covered by the tests, I added new
  tests.

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[github-actions]

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[Goooler]

I have read the CLA Document and I hereby sign the CLA

[yahavi]

Hey @Goooler and thanks for your contribution!
The renaming here does create a breaking change for Kotlin users, and it's important to note that we didn't make this change with that intention in mind. Do you think there's a genuine reason to justify this modification?

[Goooler]

I believe we should follow the Gradle guidance to use a standard extension(including honoring the standard extension name), we can note this point in changelog and recommend users to use artifactory block to configure properties like the Groovy did.