BE: Chore: Bump JRE from 21.0.6 to 21.0.8

[bguijt]

• Breaking change? (if so, please describe the impact and migration path for existing application instances)

What changes did you make? (Give an overview)

Is there anything you'd like reviewers to focus on?

How Has This Been Tested? (put an "x" (case-sensitive!) next to an item)

• No need to
• Manually (please, describe, if necessary)
  Executed:

  gradle clean bootJar
  docker buildx build api --tag kafbat/kafka-ui:1.4.0 --build-arg JAR_FILE=build/libs/api-0.0.1-SNAPSHOT.jar
  docker run --rm -it kafbat/kafka-ui:1.4.0
  

• Unit checks
• Integration checks
• Covered by existing automation

Checklist (put an "x" (case-sensitive!) next to all the items, otherwise the build will fail)

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation (e.g. ENVIRONMENT VARIABLES)
• My changes generate no new warnings (e.g. Sonar is happy)
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged

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