Replace Guava classes

[pks-1981]

Replace Guava classes
com.google.common.base.Preconditions.checkNotNull -> Objects.requireNonNull
com.google.common.base.Charsets.UTF_8 -> java.nio.charset.StandardCharsets.UTF_8
com.google.common.base.Objects -> java.util.Objects

References

#2111

Submitter checklist

• The PR request is well described and justified, including the body and the references
• The PR title represents the desired changelog entry
• The repository's code style is followed (see the contributing guide)
• Test coverage that demonstrates that the change works as expected
• For new features, there's necessary documentation in this pull request or in a subsequent PR to wiremock.org

[pks-1981]

@tomakehurst @timtebeek
Hello
Plz review it

[tomakehurst]

I'm not keen on replacing firstNonNull with Optional from a performance perspective since the former doesn't create a redundant object.

In general also, I think the goal here isn't to eliminate Guava completely. If it's providing something useful that doesn't have a direct JDK equivalent then let's keep it.