Add dependabot ignore rule for JUnit 6+ (requires Java 17)

[Copilot]

Summary

This PR adds a rule to .github/dependabot.yml to prevent Dependabot from proposing updates to JUnit 6 or later versions.

Context

The project currently:

• Uses JUnit 5.13.4 (defined in dependencyManagement/build.gradle.kts)
• Compiles with Java 17 toolchain but targets Java 8 (release.set(8) in ai.java-conventions.gradle.kts)

JUnit 6, when released, is expected to require Java 17+ as a minimum runtime version, which would be incompatible with this project's Java 8 target.

Changes

Added an ignore rule for org.junit:junit-bom to block versions [6,):

- dependency-name: "org.junit:junit-bom"
  # junit 6 requires Java 17
  versions: [ "[6,)" ]

This follows the same pattern already established in the file for other dependencies with Java version requirements:

• logback 1.4+ requires Java 11+
• mockito 5 requires Java 11
• spring-boot 3 requires Java 17

Impact

Dependabot will continue to propose updates to JUnit 5.x versions but will not create PRs for JUnit 6 or later, preventing accidental updates that would break Java 8 compatibility.

Original prompt

add a rule to dependabot.yml to prevent updating to JUnit 6 because it requires Java 17+

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