Remove usage of LogStash::SHUTDOWN

[andsel]

With the removal of Ruby Execution Engine elastic/logstash#12517 the LogStash::SHUTDOWN was removed.
This plugin starts failing on 8.0.

Build scan:
https://app.travis-ci.com/github/logstash-plugins/logstash-output-nagios_nsca/jobs/538358481#L480

Repro line:
NA

Reproduces locally?:
Yes

Applicable branches:
master

Failure history:

Failure excerpt:

logstash_1_d54db825a42b | Failures:
logstash_1_d54db825a42b | 
logstash_1_d54db825a42b |   1) LogStash::Outputs::NagiosNsca send send the event to nagios
logstash_1_d54db825a42b |      Failure/Error: return if event == LogStash::SHUTDOWN
logstash_1_d54db825a42b |      
logstash_1_d54db825a42b |      NameError:
logstash_1_d54db825a42b |        uninitialized constant LogStash::SHUTDOWN
logstash_1_d54db825a42b |        Did you mean?  LogStash::ShutdownSignal
logstash_1_d54db825a42b |      # ./lib/logstash/outputs/nagios_nsca.rb:69:in `receive'
logstash_1_d54db825a42b |      # ./spec/outputs/nagios_nsca_spec.rb:26:in `block in <main>'

[andsel]

/cc @roaksoax for prioritize

[Hellnino18]

Hi there !

What about this issue ? Do you plan to fix this problem with any ETA ? I can't use nagios_nsca output anymore since I upgraded to Elastic 8.4.3.

Thanks for your help (Elastic support told me to contact the dev platform about this issue).