[BREAKING] Upgrade Gradle, Kotlin and Interval calculations

[kkostov]

This PR brings in the following improvements:

• Gradle has been upgraded from 8.8 to 8.13
• Kotlin 2.2.21 is now the minimum required version
• Minimum supported SDK is now 23 (from 21)
• Adopted import kotlin.time.Clock instead of import kotlinx.datetime.Clock (addresses NoClassDefFoundError with kotlinx.datetime 0.7.1 #90)
• Fixed "used in last 30 days" calculation to align with the SwiftSDK

Before:

SDK	Method	Example (Feb 15)
SwiftSDK	Fixed 30-day window	Jan 16 → Feb 15 (30 days)
KotlinSDK	Calendar month subtraction	Jan 15 → Feb 15 (31 days)

After:

SDK	Method	Example (Feb 15)
SwiftSDK	Fixed 30-day window	Jan 16 → Feb 15 (30 days)
KotlinSDK	Fixed 30-day window	Jan 16 → Feb 15 (30 days)