fix: non-public main detection (JEP 512)

[warcholjakub]

Fixes a case where:

public class Test {
  protected static void main(String[] args) {
    System.out.println("hello");
  }
}

would be classified as a non-JEP512 main method.

Related: #4425 (review)

Checklist

• tested the solution locally and it works
• ran the code formatter (scala-cli fmt .)
• ran scalafix (./mill -i __.fix)
• ran reference docs auto-generation (./mill -i 'generate-reference-doc[]'.run)

How much have your relied on LLM-based tools in this contribution?

Minimally

How was the solution tested?

Automated tests

[Gedochao]

lgtm