Update JVM target to 11 in Gradle build configuration (#394)

Update JVM target to 11 in Gradle build configuration

## Motivation and Context
Drop the obsolete JVM 1.8 target to keep up with up-to-date tools, like
Kotest

## How Has This Been Tested?
CI

## Breaking Changes
The minimum JVM version is now 11

## Types of changes
<!-- What types of changes does your code introduce? Put an `x` in all
the boxes that apply: -->
- [ ] Bug fix (non-breaking change which fixes an issue)
- [ ] New feature (non-breaking change which adds functionality)
- [x] Breaking change (fix or feature that would cause existing
functionality to change)
- [x] Documentation update

## Checklist
<!-- Go over all the following points, and put an `x` in all the boxes
that apply. -->
- [x] I have read the [MCP
Documentation](https://modelcontextprotocol.io)
- [x] My code follows the repository's style guidelines
- [x] New and existing tests pass locally
- [ ] I have added appropriate error handling
- [ ] I have added or updated documentation as needed

## Additional context
<!-- Add any other context, implementation notes, or design decisions
-->

README.md

@@ -5,7 +5,7 @@
 [![Build](https://github.com/modelcontextprotocol/kotlin-sdk/actions/workflows/build.yml/badge.svg?branch=main)](https://github.com/modelcontextprotocol/kotlin-sdk/actions/workflows/build.yml)
 [![Kotlin](https://img.shields.io/badge/kotlin-2.2+-blueviolet.svg?logo=kotlin)](http://kotlinlang.org)
 [![Kotlin Multiplatform](https://img.shields.io/badge/Platforms-%20JVM%20%7C%20Wasm%2FJS%20%7C%20Native%20-blueviolet?logo=kotlin)](https://kotlinlang.org/docs/multiplatform.html)
-[![JVM](https://img.shields.io/badge/JVM-1.8+-red.svg?logo=jvm)](http://java.com)
+[![JVM](https://img.shields.io/badge/JVM-11+-red.svg?logo=jvm)](http://java.com)
 [![License](https://img.shields.io/badge/License-MIT-yellow.svg)](LICENSE)
 
 Kotlin Multiplatform implementation of the [Model Context Protocol](https://modelcontextprotocol.io) (MCP),