Upgrade Java version references from Java 8/17 to Java 21

[devin-ai-integration]

Description of what I changed

Updates all outdated Java version references across the codebase to reflect that Java 21 is the minimum required version:

• OpenmrsUtil.validateJavaVersion(): Rewrote the validation logic to properly enforce Java 21 as the minimum. The old implementation used a regex (1\\.[0-7]\\.(.*)) that only caught Java 7 and earlier, which was ineffective even for Java 8 enforcement. The new implementation parses the major version number from java.version, handling both the legacy 1.x.y scheme (Java 8 and earlier) and the modern x.y.z scheme (Java 9+), and rejects anything below 21.
• README.md: Updated the documented minimum JDK requirement from version 8 to version 21.
• pom.xml: Removed the java17 Maven profile. Since Java 21 is now the minimum, the Java 17 compatibility profile for Mockito/ByteBuddy is no longer needed. The java21 and java24 profiles are retained.
• JavaVersionTest.java: Renamed test method to reflect the Java 21 requirement.

Important items for review

1. Version parsing logic in validateJavaVersion() — Verify the parsing handles edge cases correctly (e.g., 21.0.1, 21-ea, 1.8.0_292). There are no unit tests for specific version string formats; the existing test only validates the happy path on the current JVM.
2. Removal of Java 17 Maven profile — This is a breaking change for anyone building on Java 17. Confirm no CI pipelines or downstream consumers still rely on Java 17 builds.
3. Test was not run locally — The dev environment only has Java 17, so JavaVersionTest would fail locally (expected, since 17 < 21). CI must run on Java 21+ for the test suite to pass.

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.

Note: mvn clean package was not run locally because the dev environment only has Java 17. The updated validateJavaVersion() will correctly reject Java 17, so the test suite requires Java 21+ to pass. CI should validate this.

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Link to Devin run: https://app.devin.ai/sessions/e7f727e392644473ab6aaf56e4456fab
Requested by: @dillonvargo

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✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no potential bugs to report.

View in Devin Review to see 3 additional findings.