[JENKINS-65161] Remove usage of Digester taken from Jenkins Core

[MRamonLeon]

This is a follow up PR for jenkinsci/jenkins#5320 tracked by https://issues.jenkins.io/browse/JENKINS-65161

We're removing Digester from Jenkins core because it's not used there. This PR adds Digester to this plugin and builds the object to parse xmls safely.

I can't validate the changes because a repository is no longer available. Any help about where to get those dependencies from would be appreciated:

<repository>
   <id>praqma-repo</id>
   <url></url>
</repository>

Desired reviewers per https://github.com/jenkins-infra/repository-permissions-updater/blob/master/permissions/plugin-clearcase-ucm-plugin.yml:
@MadsNielsen, @Praqma

Additional reviewers:
@alecharp @rsandell @olamy @bitwiseman @car-roll

• Make sure you are opening from a topic/feature/bugfix branch (right side) and not your master branch!
• Ensure that the pull request title represents the desired changelog entry
• Please describe what you did
• Link to relevant issues in GitHub or Jira
• Link to relevant pull requests, esp. upstream and downstream changes
• Ensure you have provided tests - that demonstrates feature works or fixes the issue

[alecharp]

@ewelinawilkosz sorry to ping you directly here, but could you help this PR to be merged here? The plugin cannot be built locally because the Praqma Maven Repository disappeared and I couldn't find it under eficode domaine.

If you (or your colleagues) cannot maintain this plugin, could it be mark as up for adoption? Thank you for any help you can provide here.

[atombrella]

@alecharp We'll mark the plugin for adoption. Thanks for highlighting this.

[MRamonLeon]

@atombrella you should have permissions now. I asked to give you the permissions on IRC. If not, ask in the IRC channel that maybe some administrator has to trigger the bot to give you the permissions on GH. https://www.jenkins.io/projects/infrastructure/ircbot/#github-repo-management

[atombrella]

@MRamonLeon Could you have a look at making this mergeable? Somehow this fork wasn't updated with the commits from praqma, which unfortunately result in the code in this repository not being able to build. It should do that now.

Then I'll release a new version today.

[MRamonLeon]

Thank you @atombrella for your work here to get it released.