Upgrade minimum Java version from 17 to 21

[dillonvargo]

Description of what I changed

This PR upgrades the minimum required Java version from 17 to 21 across the codebase:

• pom.xml: Removed the Java 17 Maven profile since it's no longer needed. The Java 21 and Java 24 profiles remain for version-specific dependency management.
• OpenmrsUtil.java: Rewrote validateJavaVersion() to properly check for Java 21 minimum. The previous implementation only rejected Java 7 and earlier using an outdated regex pattern. The new implementation parses the major version from both old-style (1.x.y) and new-style (21.0.1) version strings.
• README.md: Updated documentation to reflect Java 21 as the minimum required version.
• startup.sh: Simplified the debug port configuration by removing the Java 8 conditional branch, since Java 21+ always requires the *: prefix for the debug address.

The Dockerfile was verified and already correctly uses Java 21.

Issue I worked on

see https://issues.openmrs.org/browse/TRUNK-

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.

Items for reviewer attention

1. Version parsing logic in OpenmrsUtil.java - Please verify the parsing handles all expected Java version string formats correctly (e.g., "21.0.1", "21", "21-ea", "1.8.0_xxx")
2. No unit tests added - The validateJavaVersion() method change could benefit from test coverage for the new parsing logic

---

Requested by: Dillon Vargo (@dillonvargo)
Link to Devin run: https://app.devin.ai/sessions/720a2eb76f6c41c1b2e02d66b76475e5

[devin-ai-integration]

🤖 Devin AI Engineer

I'll be helping with this pull request! Here's what you should know:

✅ I will automatically:

• Address comments on this PR. Add '(aside)' to your comment to have me ignore it.
• Look at CI failures and help fix them

Note: I can only respond to comments from users who have write access to this repository.

⚙️ Control Options:

• Disable automatic comment and CI monitoring