refactor: split ktor adapter into package

[darwin67]

Summary

Extracts the Ktor route integration out of the core SDK and into a new publishable adapter module so core logic stays framework-agnostic and adapters import the SDK.

Changes

• Adds the new inngest-ktor-adapter module and moves com.inngest.ktor route handling and tests into it.
• Removes Ktor server dependencies from the core inngest artifact.
• Updates the Ktor test server to depend on the adapter package.
• Registers inngest-ktor-adapter in release scripts, release PR path ignores, docs, and Maven Central badges.
• Splits CI so core/Ktor run across the Java 8, 11, 17, 21, and 25 matrix while Spring Boot keeps its Java/Spring matrix.
• Runs CI jobs on Depot instances per the branch update.

Validation

• ./gradlew help
• make test-core
• make test-ktor
• make test
• make lint
• git diff --check

Related

• Follow-up to the Ktor adapter breakout discussed from PR Break out ktor adapter as a optional feature #44