Adapt the plugin to be compatable with future versions of Guava

[jtnord]

mostly by replacing all guava occurences with non guava as it is not
needed at all.
However the Multimap usage remains as there is no corresponding
equivallent and this API is not going to change between guava 11 and
guava 31.

for details see https://groups.google.com/g/jenkinsci-dev/c/aYUJ4VuOuVc/m/I9IEEzovAgAJ

@olamy

• 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

[rsandell]

Same here

Suggested change

	List<T> list = new ArrayList<>(collection);
	list.addAll(Arrays.asList(ts));
	List<T> list = new ArrayList<>(collection.size() + ts.length);
	list.addAll(collection);
	list.addAll(Arrays.asList(ts));

[olamy]

LGTM (very nitpicking comments for test classes and not mandatory to change)

[jtnord]

@olamy I have addressed the 2 comments. Would you be able to merge and release this when the build passes?

Thanks!