Update minimum Java version references from Java 8 to Java 21

[devin-ai-integration]

Description of what I changed

Updated the two remaining references to Java 8 as the minimum supported version to reflect Java 21:

1. OpenmrsUtil.validateJavaVersion() — Replaced the old regex check (1\\.[0-7]\\.(.*)) with version-parsing logic that handles both the legacy 1.x.y scheme (pre-Java 9) and the modern x.y.z scheme (Java 9+). The method now throws an APIException if the detected major version is below 21.

2. README.md — Updated the prerequisite text from "minimum version 8" to "minimum version 21".

Things for reviewers to look at closely

• Version string parsing edge cases: The new parsing handles 1.x.y, x.y.z, x-ea, and bare x formats. Verify this covers all realistic java.version values your environments produce.
• NumberFormatException risk: Unlike the old regex (which silently passed on unrecognized formats), Integer.parseInt will throw if java.version is unexpectedly malformed. This is a minor behavior change.
• No new unit tests for the parsing logic itself. The existing JavaVersionTest only calls validateJavaVersion() on the running JVM (Java 21 in CI), so it will still pass — but it doesn't exercise the rejection path or edge-case version strings.

Issue I worked on

No JIRA issue — this is a documentation/config alignment change. The project's pom.xml already targets Java 21 (<javaCompilerVersion>21</javaCompilerVersion>) and CI uses amazoncorretto-21.

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.

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Link to Devin run: https://app.devin.ai/sessions/9bfcbad15326404793a4355072703927
Requested by: @dillonvargo

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✅ Devin Review: No Issues Found

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

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