Ensure compatibility with Java 21.

[FrankenApps]

See this flutter issue and this PR for context.

This ensures that no warnings are emitted when building this package with Android Studio Ladybug / Java 21.

I contemplated setting

compileOptions {
    sourceCompatibility = JavaVersion.VERSION_17
    targetCompatibility = JavaVersion.VERSION_17
}

instead, because a lot of other popular packages require Java 17 nevertheless (for example see here or here), but ultimately decided to stick with Java 11 for improved compatibility for now, just like the official packages.