rename bootstrap environment module to differentiate from logstash-core environment

Per #5361 discussion, we should rename the bootstrap `Environment` module https://github.com/elastic/logstash/blob/ec95659dbda5792295b0d330024413fec5983901/lib/bootstrap/environment.rb to differentiate from the logstash-core `Environment` module https://github.com/elastic/logstash/blob/ec95659dbda5792295b0d330024413fec5983901/logstash-core/lib/logstash/environment.rb

The idea is what is defined in the bootstrap environment is only relevant and usable when in the context of launching the logstash app and should not be used in logstash-core code that could run outside the context of launching logstash, for example when running a plugin specs which includes the logstash-core gem.

I will also followup with another issue to remove launching specific code like `Runner` outside logstash-core.

Start with lib/bootstrap/environment.rb and logstash-core/lib/logstash/environment.rb, then trace references to both Environment modules, especially from bootstrap startup and plugin specs using the logstash-core gem. Done means the bootstrap module has a distinct name and logstash-core code does not depend on launching-specific environment behavior.

Written by the indexing model from the issue text.