Edit: Just upgrade to Spring Boot 2.5.1 where this is resolved.

Old answer below:

I managed to fix this for now by adding exclusions to my spring-boot-starter-security dependency which apparently defined this Kotlin dependency.

<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-security</artifactId>
    <exclusions>
        <exclusion>
            <groupId>org.jetbrains.kotlin</groupId>
            <artifactId>kotlin-stdlib-jdk8</artifactId>
        </exclusion>
        <exclusion>
            <groupId>org.jetbrains.kotlin</groupId>
            <artifactId>kotlin-stdlib</artifactId>
        </exclusion>
        <exclusion>
            <groupId>org.jetbrains.kotlin</groupId>
            <artifactId>kotlin-reflect</artifactId>
        </exclusion>
    </exclusions>
</dependency>