Avoid SuppressWarnings of "this-escape" in Java Setting hierarchy

[andsel]

With #16496 and commit 5ca0686 in #16490 were introduced some @SupressWarning annotations to avoid errors with JDK 21 like:

/buildkite/builds/bk-agent-prod-k8s-1728654414425952824/elastic/logstash-pull-request-pipeline/logstash-core/src/main/java/org/logstash/settings/BaseSetting.java:105: warning: [this-escape] possible 'this' escape before subclass is fully initialized
--
  | validate(defaultValue);
  | ^pileJava
  | error: warnings found and -Werror specified
  | 1 error
  | 1 warning
  |  
  | > Task :logstash-core:compileJava FAILED
  |  
  | FAILURE: Build failed with an exception.

As per comment #16490 (review) this issue is to ask for removal of such annotations and leverage static method instead of calling instance methods on non-completely initialised instances.

[andsel]

Shaped the rough idea in #16543 but still coerce method is harder to avoid

logstash/logstash-core/src/main/java/org/logstash/settings/Coercible.java

Line 31 in 6064587

T coercedDefault = coerce(defaultValue);