Kottle 2.x, which would add support for Kotlin 1.4, has yet to be released (autaut03/kottle#36). Until it is, there's a temporary fix for running it in a development environment.

In your build.gradle file, exclude kottle from the liblib transitive dependencies:

// replace this:
implementation "com.teamwizardry.librarianlib:librarianlib-core:$liblib_version"
// with this:
implementation("com.teamwizardry.librarianlib:librarianlib-core:$liblib_version") {
    exclude group: 'kottle', module: 'Kottle' 
}

Then download the slim Kottle jar from the liblib repository and add it to the runtime mods directory (e.g. run/mods/Kottle-2.0.0-slim.jar)

In 1.16+ we're using Kotlin for Forge, so this isn't an issue any more.