refactor LogStash::PluginManager::Update#validate_major_version

this method https://github.com/elastic/logstash/blob/028d76497ccef3a8d11a3528cdf6b99e8d10f070/lib/pluginmanager/update.rb#L70-L80 introduced by PR #3423 has a few issues:
- this method, like all other "lower level" methods which directly interact with Rubygems should be in the `util.rb` module.
- we should avoid, as much as possible, lazy require calls.
- I don't think we have prior-art for introducing an interactive Y/N in the logstash UI and I don't think this has been properly discussed in terms of strategy. what about automated updates? did we consider instead to introduce something like a `--force` options?
- the `validate_major_version` method name is confusing, you actually have to read the code to understand it.