docs: clarify Kotlin 2.3+/API 2.2 and Java 25/target-17 versioning per Spring Boot 4.x guidance

[Copilot]

Documentation inconsistently described the Kotlin and Java versioning strategy — conflating compiler version with language/API compatibility, and understating the Java build requirement (listing Java 17+ when Java 25 is actually required).

Changes

• AGENTS.md — Tech Stack line now encodes all four version axes in one place:

  Kotlin 2.3+ (language/API compatibility: 2.2; JVM target: 17; compile/run: JDK 25 — per Spring Boot 4.x guidance)

• docs/KOTLIN.md — New Compiler Settings section at the top explains the 2.3+ compiler / 2.2 language+API split and its practical implication (2.2 stdlib APIs, automatic 2.3+ compiler improvements).

• README.md — Build requirements corrected to Java 25; Technology section rewritten in active voice with full version detail.

• .github/CONTRIBUTING.md — Prerequisites updated to reflect API compatibility target and Spring Boot 4.x rationale; passive voice fixed in the Testing section.

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[afranken]

👍

[Copilot]

Pull request overview

This PR updates project documentation to consistently describe the versioning strategy across Kotlin compiler vs Kotlin language/API compatibility, JVM bytecode target, and the required JDK for building—aligned with Spring Boot 4.x guidance.

Changes:

• Add a “Compiler Settings” section to clarify Kotlin 2.3+ compiler usage with Kotlin language/API compatibility pinned to 2.2 and JVM target 17.
• Update build prerequisites to reflect JDK 25 as the build requirement.
• Rewrite/normalize technology stack statements across README/AGENTS/CONTRIBUTING for consistency.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 5 comments.

File	Description
docs/KOTLIN.md	Adds explicit Kotlin compiler vs language/API vs JVM target guidance.
README.md	Updates build requirements and rewrites technology/version details.
AGENTS.md	Encodes Kotlin/JVM/JDK version axes in the Tech Stack line.
.github/CONTRIBUTING.md	Updates prerequisites wording and adjusts Testing section voice.

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[Copilot]

The Technology bullet says Spring Boot “4.0”, but the repo is using a 4.0.x patch release (pom.xml sets spring-boot.version to 4.0.2, and the compatibility table above uses “4.0.x”). Consider aligning this line with the actual versioning strategy (e.g., “4.0.x” or the concrete patch).

Suggested change

	- S3Mock uses Kotlin 2.3+ (language/API compatibility: 2.2) and Spring Boot 4.0
	- S3Mock uses Kotlin 2.3+ (language/API compatibility: 2.2) and Spring Boot 4.0.x