Declare default @Retention on @Incubating explicitly

[izeye]

This PR removes explicit default @Retention declarations.

See spring-projects/spring-framework#35027

[shakuzen]

I'm not sure how common knowledge the default retention policy is. I don't know that (or have forgotten that), for example. In that sense, it might actually be nice it is explicitly there when reading the code, even if it doesn't change anything programmatically. @jonatan-ivanov thoughts?

[izeye]

@shakuzen Thanks for the feedback!

I'm okay with either way.

If we want to keep the default, the following needs to be updated instead for consistency:

micrometer/micrometer-core/src/main/java/io/micrometer/core/annotation/Incubating.java

Lines 22 to 32 in b626908

	/**
	* This is a feature that is experimental and may yield a breaking change in a minor
	* release.
	*/
	@Documented
	@Target({ ElementType.METHOD, ElementType.TYPE, ElementType.CONSTRUCTOR })
	public @interface Incubating {
	String since();
	}

[jonatan-ivanov]

I don't have a strong opinion but I'm leaning towards spelling this out explicitly.
@izeye, if it's ok with you, would you mind do the change on @Incubating instead?

[izeye]

@jonatan-ivanov Sure, I updated this as suggested.

[jonatan-ivanov]

Thank you!