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)

Background

Our samples server several purposes:

1. simple copy/paste for users to run w/
2. integration tests for project
3. matrix compatibility tests for project

The Maven samples should be kept simple and static in order to serve purpose 1.
The Gradle samples can be more complex to serve purpose 3.
Both Maven and Gradle samples serve purpose 2.