Support Gradle configuration cache and parallel execution

[perNyfelt]

Summary

• refactor processGmd into a task with declared inputs, outputs, and classpath
• remove Project access and dependency resolution from task execution
• declare configuration-cache support in Plugin Portal metadata
• update signing configuration to avoid the deprecated Project.getProperties() API
• bump the project version to 3.1.1-SNAPSHOT
• update the Gradle plugin release notes

Why

The previous implementation captured Project in the task action and changed repositories while executing. That prevented configuration-cache support and made parallel execution unsafe. The new task resolves project configuration up front and uses injected Gradle services during execution.

Validation

• ./gradlew clean build publishPlugins --validate-only --warning-mode all -Ppublish.version=3.1.1
• mvn clean install
• TestKit verifies the first configuration-cache run stores the cache and the second run reuses it with --parallel

The task is explicitly not build-cacheable because generated PDF metadata may be non-deterministic.