Collect public Maven repos beyond Maven central

Some lists:
- https://mvnrepository.com/repos tracks ~ 2000 public maven repos

Plain repos:
- jetbrains
- https://maven.pkg.jetbrains.space/public/p/kotlinx-html/maven
- https://maven.pkg.jetbrains.space/kotlin/p/kotlin/dev
- https://packages.confluent.io/maven/

Nexus repos:
- https://repo.hortonworks.com/ and in particular
- https://repo.hortonworks.com/service/rest/repository/browse/releases/ and has some index at https://repo.hortonworks.com/repository/releases/files.txt
- https://repository.jboss.org/nexus/content/repositories/ea/

Artifactory repos:
- https://packages.atlassian.com/mvn/maven-atlassian-external/
- Spring
- https://repo.spring.io/ui/native/plugins-release/
- https://repo.spring.io/libs-milestone/
- https://linkedin.jfrog.io/artifactory/open-source/