Get rid of deprecated plugin warning in Jenkins

Since we've update Jenkins to 2.253-slim and some plugins, see NXBT-3372, we get this warning about the deprecated plugin "Pipeline: Declarative Agent API", see attached screenshot.

The problem is that we are currently installing the list of plugins that come from the gcr.io/jenkinsxio/jenkinsx image, then only we override some of them with the latest version.
Thus, for instance, the following set of plugins get installed with an old version:

pipeline-model-api:1.4.0
pipeline-model-declarative-agent:1.1.1
pipeline-model-definition:1.4.0
pipeline-model-extensions:1.4.0

when running

RUN /usr/local/bin/install-plugins.sh < /usr/share/jenkins/ref/plugins.txt
...
13:51:19  Downloading plugin: pipeline-model-definition from https://updates.jenkins.io/download/plugins/pipeline-model-definition/1.4.0/pipeline-model-definition.hpi
...

and this is how we get pipeline-model-declarative-agent, as a transitive dependency.

A possible solution is to not copy/install the list of plugins coming from the jenkinsx image and build our own, complete but minimal and up-to-date list of plugins.