chore: move off guava; low hanging fruits

[cstamas]

Many of Guava helpers were concieved before modern Java language features.

This PR moves to "pure Java" counterpart where possible.

Also, Guava is not among dependencies, is transitive dependency, so next is worth to consider it as direct dependency, as it is being used.

[netomi]

I did some more replacements as modern java has a counterpart.

Also added an explicit dependency to guava, it was only pulled in as a transitive dependency from the google cloud sdk though we use it in various places in a way that is not so easy to replace.

[cstamas]

LGTM. On thing though: I did not want to add Guava to deps, as that would mean (to someone just looking at the build) that Guava is "free to use". I'd somehow emphasize to still stay away from it, ideally fully replace it and "demote" it into real transitive dependency.

Edit: maybe add some comment to it like "use only as last resort; avoid otherwise"?

[gnugomez]

LGTM nice cleanup!

[netomi]

as long as we explicitly use the library in our code we should list it as a dependency. That also gives us the chance to easily bump it if necessary.

We should clarify in the CONTRIBUTING.md though what is acceptable and what not.

Maybe we should create a specific AGENTS.md file for the server where we specify what is acceptable and what not to help agents which will do the majority of coding anyways.

For the webui I already added one.