Improve coverage of Java 17 and 25 w/ the Maven and Gradle sample apps

[onobc]

• Update checks-samples.yml to run Gradle samples using Java 17 and 25
• Add check-samples-maven.yml to run Maven samples using Java 17 and 25
• Simplify ci-pr.yml (remove running of samples)
• Add SpringBoot version and Spring gRPC version to workflow matrix (for Gradle samples)