feat: BigQuery Storage v1beta1 API migration guide

[benhxy]

Description

Fix b/505001153

BigQuery Storage team is planning to deprecate v1beta1 API. This PR adds a migration guide from v1beta1 to v1.

Checklist

Testing

• I have tested this change on a live environment and verified it works as intended.
• Tests pass: mvn clean verify required
• Lint passes: mvn -P lint checkstyle:check required
• Static Analysis: mvn -P lint clean compile pmd:cpd-check spotbugs:check advisory only

Compliance & Style

• I have followed Sample Format Guide
• pom.xml parent set to latest shared-configuration
• Appropriate changes to README are included in PR
• This sample adds a new sample directory, and I updated the CODEOWNERS file with the codeowners for this sample
• This sample adds a new Product API, and I updated the Blunderbuss issue/PR auto-assigner with the codeowners for this sample

Post-Approval Actions

• Please merge this PR for me once it is approved

[gemini-code-assist]

Code Review

This pull request introduces a migration guide for transitioning Java code using the BigQuery Storage API from version v1beta1 to v1. The guide details key changes and provides code comparisons for client initialization, creating read sessions, and reading rows. The review feedback suggests adding missing import java.io.IOException; statements to the Java code snippets to ensure they are complete and copy-pasteable.