Refine KotlinDetector usages and implementation

[sdeleuze]

Initially Spring Framework did not required kotlin-reflect in a mandatory way, it does now for some time as documented in the reference documentation, as it is almost impossible to provide a reasonable Spring experience without it. Also we expect that #21546 will largely improve kotlin-reflect efficiency.

Since KotlinDetector#isKotlinType and KotlinDetector#isSuspendingFunction already have fast and classpath-safe checks of the presence of Kotlin in the classpath, that means there are various KotlinDetector#isKotlinReflectPresent invocations that can be removed from the codebase.

There are also some useless KotlinDetector#isKotlinPresent invocations that we can remove as well.

com.fasterxml.jackson.module.kotlin.KotlinModule requires kotlin-reflect, so the related classpath check in Jackson2ObjectMapperBuilder should be adapted.

Finally, KotlinDetector implementation can be optimized.