Remove explicit dependency on slf4j implementation and rely on the api instead

[troyunverdruss]

Actual behavior (the bug)
The openapi-annotation-processor is pulling in the logback library as a dependency.

Expected behavior
The library should depend on the slf4j-api according to slf4j docs: https://www.slf4j.org/codes.html#multiple_bindings

To Reproduce
Any product's build.gradle config that specifies an alternative slf4j implementation will give you a warning that there are multiple bindings available.

Additional context
I suspect that the original authors of the library ran into this because this line excludes the org.slf4j:

https://github.com/javalin/javalin-openapi/blob/main/openapi-annotation-processor/build.gradle.kts#L18

and I think it's probably because of a conflict. According to the doc I linked above about multiple bindings, this is likely a mistake since it forces an implementation on the end-user of this library.

[dzikoysk]

openapi-annotation-processor is not a standard module - it runs only at compile-time (so basically when gradle/maven compiles your project) and should never be a part of your actual application. You may see this warning during compilation if you have multiple libs on your classpath at this time, but despite that it should not affect your code.