Improve Docker configuration: standardize JDK, add JAVA_VERSION arg, update Tomcat

[dillonvargo]

Description of what I changed

Three improvements to the Docker and CI configuration:

1. Made Java version configurable via a single JAVA_VERSION build arg in the Dockerfile. DEV_JDK and RUNTIME_JDK are now derived from it, so switching Java versions (e.g., --build-arg JAVA_VERSION=17) requires changing only one value instead of two.

2. Standardized JDK distribution references across Dockerfile and bamboo-specs/bamboo.yml. The Dockerfile already used Eclipse Temurin; the CI pipeline (bamboo.yml) used amazoncorretto-21 for image tagging in the Deploy and Release jobs. Both now consistently use eclipse-temurin-21.

3. Updated Tomcat from 11.0.11 → 11.0.18 (latest stable release) with updated SHA512 checksum.

Items for reviewer attention

• Docker ARG interpolation before FROM: JAVA_VERSION is defined in global scope and used to derive DEV_JDK/RUNTIME_JDK which feed into FROM instructions. This should work per Docker spec, but is worth verifying with a build.
• Image tag change in bamboo.yml: The JDK_IMAGE variable is used as a suffix for published Docker image tags (e.g., openmrs/openmrs-core:nightly-eclipse-temurin-21). Previously these tags used -amazoncorretto-21. Downstream consumers pulling by the old tag suffix will need to update.
• Tomcat SHA512 checksum was sourced from downloads.apache.org; please independently verify.

Issue I worked on

No JIRA issue — configuration improvement for maintainability.

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/4a84210e3aad4dbd8329f36959782d2c
Requested by: @dillonvargo

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