Remove compiler option to suppress warnings for using experimental unsigned types

[thunderbiscuit]

The following block is no longer necessary as of Kotlin 1.5 (we're currently using 1.6.10):

allprojects {
    tasks.withType<org.jetbrains.kotlin.gradle.tasks.KotlinCompile>().configureEach {
        kotlinOptions.freeCompilerArgs += "-opt-in=kotlin.ExperimentalUnsignedTypes"
    }
}

See uniffi-rs docs here and related merged PR here for more info.

I can't assign myself but @notmandatory feel free to assign me this one.

[artfuldev]

@notmandatory can we add a release target/milestone/version for this?

[artfuldev]

@thunderbiscuit I missed the part about us using a later kotlin version than 1.5 🤦

[thunderbiscuit]

Yes well to be honest I'm not sure it's enforced anywhere, but I think the following block will throw warnings at you if you're using a different version:

buildscript {
    ext.kotlin_version = '1.6.10'
    repositories {
        google()
        mavenCentral()
    }
    dependencies {
        classpath 'com.android.tools.build:gradle:7.0.4'
        classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:$kotlin_version"
    }
}

[notmandatory]

I think it's reasonable to update the README as part of this PR to require Kotlin version 1.6.x.

[notmandatory]

Fixed by #17