Use kotlinx-datetime instead of Java APIs

[MGaetan89]

Description

This PR replaces Java-specific APIs with Kotlin's ones. In particular, it:

• replaces date-related APIs with the kotlinx-datetime library (see the "Migration guide" below).
• changes the type of SpriteSheet.aspectRatio to AspectRatio (previously it was an android.util.Rational).
• removes java.io.Serializable from ImageUrl.

Warning

This PR is a breaking change compared to the previous version!

kotlinx-datetime

• ✅ It supports kotlinx-serialization out of the box. So the ISO8601DateParser and DateSerializer are no longer necessary.
• ✅ It is a multiplatform library, in case we ever want to go down that road.
• ⚠️ It is currently in , meaning that its API may change or be removed at any time. However, according to Kotlin's roadmap, it should reach the status "soon" (KT-64578), meaning that the APIs is close to finalized and breaking changes should be rare.
• ⚠️ On Android, it uses the java.time API internally. So if the min API is less than 26, core library desugaring is necessary.

Migration guide

The official documentation provides more information about kotlinx-datetime API, but here are some quick tips to help you get started:

Java API	Kotlin API	Example
Date()	Clock.System.now()
Date.time	Instant.toEpochMilliseconds()
Date.after(Date)	Instant > Instant
Date.before(Date)	Instant < Instant
Date(timestamp + offset)	Instant + Duration	Clock.System.now() + 5.minutes

Next steps

Following the merge of this PR:

• srgletterbox-android will be updated as well to use the new models → https://github.com/SRGSSR/srgletterbox-android/pull/542
• playsrg-android needs to be updated to support these changes → https://github.com/SRGSSR/playsrg-android/pull/314