Add comprehensive Java 24 upgrade plan document

[dillonvargo]

Description of what I changed

Adds JAVA_24_UPGRADE_PLAN.md — a planning document that outlines a phased approach to upgrade OpenMRS Core from Java 21 to Java 24 as the primary target. No code changes are included; this is documentation only.

The plan covers 5 phases:

1. Remove dead code — tools.jar profiles, redundant Mockito profiles, legacy .travis.yml
2. Dependency migrations — javax.mail → jakarta.mail (with specific source file changes listed), javax.servlet-api in test-module
3. Outdated library upgrades — Velocity 1.7 → 2.x, Groovy 2.4 → 4.x
4. Build/runtime config — javaCompilerVersion, Dockerfile JDK args, CI matrix, JVM arguments
5. Documentation — README minimum version, comment cleanup

Each item includes file locations, exact actions, risk levels, and dependency ordering.

Issue I worked on

N/A — this is a planning document requested ahead of implementation.

Important items for reviewer

• Verify suggested target library versions are current and mutually compatible (e.g., jakarta.mail-api:2.1.3 with angus-mail:2.0.3, Velocity 2.4.1, Groovy 4.0.24).
• Spring 6.2.x + Java 24 compatibility: The CI workflow has a comment saying "Current spring version only supports up to java 21." The plan asserts Spring 6.2.x works with Java 24 — this should be verified against Spring's official compatibility matrix before acting on Phase 4.
• Module ecosystem impact: The plan briefly notes that downstream OpenMRS modules using javax.mail or javax.servlet will break. Consider whether a more detailed module migration guide or communication plan is needed.
• Should this file live in the repo permanently? Some teams prefer plans in a wiki or issue tracker rather than committed to the codebase.
• The plan correctly excludes javax.xml.*, javax.crypto.*, javax.imageio.* from migration (they're JDK-internal, not Jakarta EE), but reviewers familiar with the codebase should confirm no other javax.* → jakarta.* migrations were missed.

Checklist: I completed these to help reviewers :)

• My IDE is configured to follow the code style of this project.
• I have added tests to cover my changes. (If you refactored existing code that was well tested you do not have to add tests)
• I ran mvn clean package right before creating this pull request and added all formatting changes to my commit.
• All new and existing tests passed.
• My pull request is based on the latest changes of the master branch.

Link to Devin run: https://app.devin.ai/sessions/4fe707b80dc44b53859baf47a63d214e
Requested by: @dillonvargo

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