Avoid lock when ecs_compatibility is explicitly specified (#16786)

Because a `break` escapes a `begin`...`end` block, we must not use a `break` in order to ensure that the explicitly set value gets memoized to avoid lock contention.

> ~~~ ruby
> def fake_sync(&block)
>   puts "FAKE_SYNC:enter"
>   val = yield
>   puts "FAKE_SYNC:return(#{val})"
>   return val
> ensure
>   puts "FAKE_SYNC:ensure"
> end
>
> fake_sync do
>   @ivar = begin
>     puts("BE:begin")
>   	break :break
>
>   	val = :ret
>   	puts("BE:return(#{val})")
>   	val
>   ensure
>     puts("BE:ensure")
>   end
> end
> ~~~

Note: no `FAKE_SYNC:return`:

> ~~~
> ╭─{ rye@perhaps:~/src/elastic/logstash (main ✔) }
> ╰─● ruby break-esc.rb
> FAKE_SYNC:enter
> BE:begin
> BE:ensure
> FAKE_SYNC:ensure
> [success]
> ~~~

(cherry picked from commit 01c8e8b)

logstash-core/lib/logstash/plugins/ecs_compatibility_support.rb

@@ -9,10 +9,11 @@ def self.included(base)
 
       def ecs_compatibility
         @_ecs_compatibility || LogStash::Util.synchronize(self) do
-          @_ecs_compatibility ||= begin
-            # use config_init-set value if present
-            break @ecs_compatibility unless @ecs_compatibility.nil?
+          # use config_init-set value if present
+          @_ecs_compatibility ||= @ecs_compatibility
 
+          # load default from settings
+          @_ecs_compatibility ||= begin
             pipeline = execution_context.pipeline
             pipeline_settings = pipeline && pipeline.settings
             pipeline_settings ||= LogStash::SETTINGS