[ci] build the release on JDK 17 and document it

The release workflow set up JDK 11 while the poms compile with source/target
17, so mvn deploy would have failed at compilation with "invalid target
release: 17" on the first tag push. CI already uses 17.

The READMEs claimed Java 11, which has been wrong since the move to 17: class
files are version 61, so an 11 runtime fails with UnsupportedClassVersionError
after resolving the dependency rather than at build time.

Author: gmtuca

.github/workflows/release.yml

@@ -11,10 +11,11 @@ jobs:
     steps:
       - uses: actions/checkout@v4
 
-      - name: Set up JDK 11
+      # Must match maven.compiler.target in the root pom (17), and the CI workflow.
+      - name: Set up JDK 17
         uses: actions/setup-java@v4
         with:
-          java-version: '11'
+          java-version: '17'
           distribution: 'temurin'
           cache: maven
           server-id: central

README.md

@@ -10,7 +10,7 @@ Java client library for the [Gradient Labs API](https://api-docs.gradient-labs.a
 - Type-safe models and enums
 - Webhook signature verification
 - Comprehensive error handling
-- Java 11+ compatible
+- Java 17+ compatible
 - Zero runtime dependencies (except Jackson)
 
 ## Spring Boot Integration
@@ -167,7 +167,7 @@ GradientLabsClient client = GradientLabsClient.builder()
 
 ## Requirements
 
-- Java 11 or higher
+- Java 17 or higher
 - Jackson 2.18+ for JSON serialization
 
 ## Documentation

gradient-labs-client/examples/spring-boot-example/README.md

@@ -14,7 +14,7 @@ This example demonstrates how to use the Gradient Labs Spring Boot Starter for z
 
 ### Prerequisites
 
-- Java 11 or higher
+- Java 17 or higher
 - Maven
 - Gradient Labs API key
 

gradient-labs-spring-boot-starter/README.md

@@ -252,7 +252,7 @@ The starter includes Spring Boot configuration metadata for IDE autocomplete sup
 
 ## Requirements
 
-- Java 11 or higher
+- Java 17 or higher
 - Spring Boot 3.2.0 or higher
 - The core `gradient-labs-client` library (automatically included as a transitive dependency)