Update to kotlinx-datetime:0.7.0 and fix binary incompatible changes.

[colinrtwhite]

kotlinx-datetime:0.7.0 completely removes kotlinx.datetime.Clock and kotlinx.datetime.Instant in favour of the kotlin.time versions (more info). This creates a couple issues:

• We currently use kotlinx.datetime.Instant in FakeFileSystem's public API. We probably have to take a binary incompatible change here and update to the new class.
• kotlin.time classes are marked with @ExperimentalTime which will propagate to our FakeFileSystem's constructor unless we add a secondary constructor: constructor() : this(Clock.System) that hides the experimental arg.
• Currently all okio consumers have to use 0.7.0-0.6.x-compat (a compat version that retains the removed classes) if they want to update to 0.7.0 and use FakeFileSystem.

[JakeWharton]

We probably have to take a binary incompatible change here and update to the new class.

I believe this was the concession when we originally took the unstable dependency. It's a test only dependency (or should be) so it's okay that we take this hit.

[colinrtwhite]

Not sure if it's worth maintaining this secondary constructor forever to avoid propagating @ExperimentalTime to FakeFileSystem(). Let me know what you think

[JakeWharton]

Seems like a good idea. We can do @JvmOverloads once this type goes stable.

[swankjesse]

Thanks for all the attention to compatibility!

[mathieularue]

Is there any chance that this will be included in an upcoming release? Thanks!

[JakeWharton]

It will be released today or tomorrow, yes.

[swankjesse]

When this build completes, the release is out. https://github.com/square/okio/actions/runs/15945624170