build(gradle): Enable consistent copy() visibility

Apply upcoming Kotlin 2.1 behavior to align the `copy()` function's
visibility to the visibility of the primary constructor for data classes.
This avoids the new compiler warning with Kotlin 2.0.20 if properties of a
private primary constructor are exposed via `copy()`, even though the
`copy()` function is not called.

Signed-off-by: Sebastian Schuberth <sebastian@doubleopen.org>

buildSrc/src/main/kotlin/ort-kotlin-conventions.gradle.kts

@@ -183,6 +183,7 @@ tasks.withType<KotlinCompile>().configureEach {
 
     compilerOptions {
         allWarningsAsErrors = true
+        freeCompilerArgs = listOf("-Xconsistent-data-class-copy-visibility")
         jvmTarget = maxKotlinJvmTarget
         optIn = optInRequirements
     }