Added archived attribute in GHRepository

[recena]

• archived attribute was missed and can be interesting in many cases
• Updated parent POM from 17 to 20
• Addressed a Apache Maven warning

[WARNING] 
[WARNING] Some problems were encountered while building the effective model for org.kohsuke:github-api:jar:1.95-SNAPSHOT
[WARNING] 'build.plugins.plugin.version' for org.apache.maven.plugins:maven-surefire-plugin is missing. @ line 37, column 15
[WARNING] 
[WARNING] It is highly recommended to fix these problems because they threaten the stability of your build.
[WARNING] 
[WARNING] For this reason, future Maven versions might no longer support building such malformed projects.
[WARNING] 

@kohsuke, could you take a look? Thanks.

[brian-mcnamara]

@recena Thanks for this, I was actually just about to implement this as I need it too! Thank you!

@kohsuke I would love to use this for our project. Can we create a release?

[nielsutrecht]

@kohsuke +1 from me, I'm running into an issue where I'm mass creating pull requests that are failing on archived repositories.