dependencies {
    implementation(libs.plugins.kotlin.jvm.toDep())
}

This isn’t advisable, it messes with the dependencies and Kotlin versions that Gradle depends on for Gradle-code.

I typically keep my Gradle code aligned with the Gradle versions that it targets (there is not yet any Gradle version that uses Kotlin 1.9) and bump the Gradle version occasionally (perhaps that brings a Kotlin version change as well). iirc there is a way to change the Kotlin language level for your Gradle code - on recent Gradle versions you could use some experimental features from 1.9 (such as data objects).

This is how the Kotlin language level is changed for my non-Gradle code, perhaps this would work in your build-logic/build.gradle.kts:

tasks.withType<KotlinCompile>().configureEach {
    compilerOptions {
        languageVersion.set(KotlinVersion.KOTLIN_1_9)
    }
}

and consider using

compileOnly

such that consumers of your plugin could provide their own version, as the plugin could presumably run on different Gradle versions.

I have this in a settings.gradle for version catalog:

plugin("gradle.kotlin.dslPlugins" to "org.gradle.kotlin.kotlin-dsl:$expectedKotlinDslPluginsVersion")
            plugin("jetbrains.kotlin.jvm" to "org.jetbrains.kotlin.jvm:$embeddedKotlinVersion")

lol, and this in a build script that should be using the version catalog:

compileOnly(plugin("org.gradle.kotlin.kotlin-dsl", expectedKotlinDslPluginsVersion))
    compileOnly(plugin("org.jetbrains.kotlin.jvm", embeddedKotlinVersion))

Or, I just wait until Gradle 8.3 is out containing Kotlin 1.9