Bump and add possibility to override Moshi version applied by IR plugin

[plnice]

Two commits here:

1. Bump Moshi version applied by IR plugin from 1.13.0 to 1.15.1. 1.13.0 was released on 2021-12-08 and 1.15.1 was released on 2024-01-30, I think it makes sense to apply newest version. Also 1.15.1 is already widely used throughout the MoshiX repository.
2. Add possibility to override the version in case of somebody needs older or newer one, if it's there. It should be convenient to do this by utilizing simple property in Gradle plugin extension. Moved default value to property's convention as it makes sense to set the default version value as early as possible, and then only read it.

[ZacSweers]

hmm, I'm not sure I want to do this actually. It starts to blur the line of this plugin acting as dependency management middleware. I'd be open to, instead, making the auto-application of the moshi dependency a disable-able behavior though. We intentionally only apply the minimum required version at the moment.

[plnice]

Got it - in my project we apply Moshi manually anyway to not depend on transitive dependency in our source code (the rationale of opening this PR was to remove adding 1.13.0 unnecessarily if we use 1.15.1). Let me open another PR with possibility to disable auto-application of Moshi dependency