build(deps): align Palantir Jackson to 2.21.5

[jbeckwith-oai]

Summary

• align the Jackson family used only by the Palantir Java formatter to the fixed 2.21.5 BOM
• exempt only the isolated palantir configuration from the core module's Jackson 2.14 compatibility-test force
• address Dependabot alert #101 without changing SDK runtime dependencies

This PR is intentionally stacked on #784, which updates palantir-java-format to 2.96.0. It should be reviewed and merged after #784.

Compatibility

This is build-tool-only configuration. Dependency inspection confirms:

• Palantir resolves jackson-databind and its Jackson modules at 2.21.5
• the SDK compatibility test runtime remains on Jackson 2.14.0
• the published-runtime test remains on Jackson 2.18.9
• generated Maven POMs contain neither Palantir nor Jackson 2.21.5; the published core POM still declares Jackson 2.18.9
• no SDK source, generated model, public API, or runtime configuration changes

Validation

• ./scripts/lint
• ./scripts/build
• ./scripts/test
• ./scripts/detect-breaking-changes b00633be16fdb678b3adce953c9429e1cabd0216
• ./gradlew publishAllPublicationsToLocalFileSystemRepository -PpublishLocal
• regenerated and inspected every published Maven POM after rebasing onto the current parent
• Gradle dependencyInsight checks for the Palantir, compatibility-test, and published-runtime Jackson graphs