Upgrade to Kotlin 2.x and set a new related baseline

Spring Framework 7 will use Kotlin 2.x as a new baseline, TBD if we use 2.0, 2.1, 2.2 or 2.3 as the minimal version supported, this kind of decision could be driven by #21546 requirements.

Kotlin Serialization and Kotlin Coroutines should be upgraded accordingly, related upgrade issue will be created when it is clearer what version we will use.