Add Java 8 to Java 9 Migration Strategy Document for COG-GTM/springboot-java8

[devin-ai-integration]

Summary

Adds MIGRATION_STRATEGY.md to the repository root — a comprehensive planning document for migrating this Spring Boot application from Java 8 to Java 9. This is a documentation-only PR; no source code or build files are changed.

The document contains:

• Current state analysis of build configuration (Maven vs Gradle dependency drift), all 13 source files, and 11 REST endpoints
• 5-phase migration strategy: pre-migration prep (sync Gradle, add tests) → Spring Boot 2.0.2→2.1.18 upgrade → Java 9 compilation target → validation → cleanup
• 9 Jira ticket definitions (JAVA9-1 through JAVA9-9) with type, priority, dependencies, affected files, and acceptance criteria
• Mermaid dependency graph showing ticket ordering and parallelization opportunities
• Effort summary (17 story points total) and risk assessment

Review & Testing Checklist for Human

• Verify line number references are accurate: The document cites specific lines in pom.xml (e.g., line 14 for Spring Boot version, line 38 for java.version) and build.gradle (e.g., lines 25-26 for source/targetCompatibility). Spot-check these against the actual files to ensure they haven't drifted.
• Validate Spring Boot 2.1.18.RELEASE as the right target: The document recommends this specific version for Java 9 support. Confirm this aligns with your team's constraints (e.g., if there's a reason to go to 2.2.x+ instead).
• Review the "no source code changes needed" claim: The document asserts all Java 8 APIs used are forward-compatible with Java 9. If you're aware of any edge cases in this codebase (e.g., reflection usage, sun.* internal APIs), flag them.

Notes

• The Jira ticket IDs (JAVA9-1 through JAVA9-9) are placeholder prefixes for the migration plan — they are not yet created in any Jira instance.
• The document notes the gturnquist-quoters external API endpoint (used by RestTemplate in Application.java) may be defunct — this is a pre-existing issue unrelated to the migration.

Link to Devin session: https://app.devin.ai/sessions/3d1dc8f122cb41ca98494fe237f9f4b6
Requested by: @patrickbradley-cog

[devin-ai-integration]

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