My old team used Spring and Kotlin and it was a really productive combination. Spring isn't standing still. They have first class support for Kotlin and rewriting their framework to support the graalvm.
That's not necessarily a bad thing, but usually people who are giving the JVM a "second chance" via Kotlin were initially turned off by that ceremony
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So when someone asks me "What should framework should I try for Kotlin web services?" my answer is usually Ktor.
Because if they had to ask me, they probably aren't going to be that in love with Spring Boot (people coming from Java from example, are usually just going to default to Spring Boot)
There's everything, from the simplest possible https://sparkjava.com/, to the a little bit nicer https://javalin.io/ (which has a Kotlin wrapper), to the high performance and full framework https://vertx.io/ (which always had great multi-lingual support within the JVM), to the newer and GraalVM-friendly https://micronaut.io/ and https://quarkus.io/ ... for the Enterprise-only crowd, you still have the JEE successor, https://jakarta.ee/ and Oracle's own https://helidon.io/#/ (which likes to position itself as microservice focused, like Micronaut and Quarkus). Not to mention lower level libraries you can use, like https://www.eclipse.org/jetty/ and https://netty.io/.
Ktor has a tough fight to become dominant even in the Kotlin world.
Spring Boot is, still, definitely the most popular option, but it's still just one of very very many!
It also changes the development workflow from a maven-based project to a NPM-based project which should be more familiar to JavaScript developers.