plugin manager should not mention or consider incompatible versions of plugins to install

Logstash should ignore versions of plugins that are not compatible with the current version of Logstash.

In the following case, I am asked whether I would like to install logstash-output-kafka 4.0, which is only compatible with LS 5.x. Then it ignores that recommendation and finds the largest version of the plugin that _is_ compatible, and installs that properly.
- Version: 2.3.x
- Operating System: All
- Steps to Reproduce:
1. install logstash 2.3.1
2. run `bin/logstash-plugin update logstash-output-kafka`

```
$ bin/logstash-plugin update logstash-output-kafka
You are updating logstash-output-kafka to a new version 4.0.0, which may not be compatible with 2.0.3. are you sure you want to proceed (Y/N)?
y
Updating logstash-output-kafka

WARNING: SSLSocket#session= is not supported
Updated logstash-codec-json 2.1.3 to 2.1.4
Updated logstash-output-kafka 2.0.3 to 2.0.5
```