This commit upgrades to Kotlin 2.1.0. Two related dependencies have been
updated as well: Kotlin Coroutines to 1.10, and Kotlin Serialization to
1.8.
As of Kotlin 2, it is no longer possible to have a Java type and a
Kotlin type with the same name. As our code samples follow that
unfortunate pattern, this commit makes sure that the Kotlin sample code
does not depend on any of the Java counterpart and configure the kotlin
compilation plugin to ignore Java sources.
The minimum version of Gradle is 7.6.4. It bundles a version of Kotlin
that cannot compile a Kotlin build script when spring-core, compiled
with Kotlin 2.1, is on the classpath. Using Gradle 8.12 to run the DSL
tests avoids the problem.
Closes gh-45486
Co-authored-by: Andy Wilkinson <andy.wilkinson@broadcom.com>
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