feat: update functions samples

[maigovannon]

Description

Creating Java25 doc samples for Cloud Run Functions and updating some of the existing ones to Java21. These are forked changes from https://github.com/GoogleCloudPlatform/java-docs-samples/pull/10193

Checklist

• [x] I have followed Sample Format Guide
• [x] pom.xml parent set to latest shared-configuration
• [x] Appropriate changes to README are included in PR
• [x] Tests pass: mvn clean verify required
• [x] Lint passes: mvn -P lint checkstyle:check required
• [ ] Static Analysis: mvn -P lint clean compile pmd:cpd-check spotbugs:check advisory only

[gemini-code-assist[bot]]

Summary of Changes

Hello @maigovannon, I'm Gemini Code Assist[^1]! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request updates the Java compiler version for several function samples from Java 11 to Java 21. The primary purpose is to modernize the build configurations for these samples, ensuring they leverage the latest LTS features and maintain compatibility with contemporary Java development environments. This update affects the helloworld and pubsub samples, bringing their pom.xml configurations up to date.

Highlights

• Java Version Update: The maven.compiler.target and maven.compiler.source properties have been updated from Java 11 to Java 21 in the pom.xml files for the helloworld and pubsub function samples.
• Sample Modernization: This change ensures that the specified function samples are compiled and run using the latest Long-Term Support (LTS) version of Java, aligning them with current development standards.

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