Review LS startup to re-use expanded JRuby bits

[kares]

ATM, LS startup depends on jruby-complete.jar (which includes the Ruby stdlib) from logstash-core/lib/jars.
On the other hand, LS has an expanded JRuby installation located at vendor/jruby.

The JRuby installation includes the (core) vendor/jrubylib/jruby.jar and also the stdlib files, but LS is only using these for it's logstash-plugin management script.

This is confusing (run into this while trying to upgrade RubyGems - just to find out the files are not used on normal LS startup) and bin/logstash should rather re-use it's JRuby installation bits.

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NOTE: this issue is blocking a potential easy resolution for #12077 (upgrading RGs to latest)

[kares]

this landed in time for 8.1: #13159