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

[MRamonLeon]

This is a follow up PR for jenkinsci/jenkins#5320 tracked by 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 had to fix some warnings linter were yelling.

No maintainers listed in https://github.com/jenkins-infra/repository-permissions-updater/blob/master/permissions/plugin-cmvc.yml

Pinging the contributors: @ndeloof @alanharder
Additional reviewers: @alecharp @olamy @rsandell @car-roll @bitwiseman @jtnord

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[rsandell]

Though while updating you should probably have taken the few extra seconds to make the digester safe as well.

[batmat]

we must use the safety settings

[alecharp]

a small question about the transient field.