Upgrade to Spring Boot 4 and Jackson 3

[danlysiak]

JIRA link (if applicable)

N/A

Change description

Upgrades CCD Data Store API to Spring Boot 4.0.7 and Spring Cloud 2025.1.2 while preserving existing service behaviour.

Key changes:

• migrates CCD-owned JSON processing, generated models, Spring MVC payloads, and Elasticsearch JSONP mapping to Jackson 3
• adopts the Boot 4 modular starters and corresponding Spring Framework, Security, Hibernate, Flyway, WireMock, and test APIs
• retains AM client 1.60.0 through a consumer-side parameter-names exclusion and Jackson 3 mixin, avoiding an unpublished AM dependency
• isolates Gson to supported JJWT and raw AAT Feign codecs where upstream Jackson codecs remain Jackson 2-based
• updates Application Insights integration and adds focused regressions for serialization, AM documents, Feign converters, and telemetry

Change classification

Area	Classification	Rationale
Boot 4 / Cloud / modular starters	Required	Platform major-version migration
Jackson 3 source and generated models	Required	Boot 4 application JSON stack
HMCTS clients, WireMock, Rest Assured, BEFTA	Compatibility	Align dependent libraries and test infrastructure
AM mixin and Feign converter configuration	Behaviour-preserving	Preserve released-client deserialization and cold-start converter behaviour
Telemetry and hypermedia configuration	Behaviour-preserving	Preserve application behaviour after removed/changed auto-configuration

Validation

• ./gradlew test checkstyleMain checkstyleTest checkstyleContractTest checkstyleAat bootJar
  • 4,118 tests passed; 0 failures; 0 errors; 52 skipped
  • all four Checkstyle source sets passed
  • executable Boot jar built successfully
• integration suite: 25 tests; 0 failures/errors; 8 skipped
• git diff --check passed
• dependency/source/bytecode audits found no Jackson 2 core/databind/module references in CCD-owned code
• independent migration-guide reviews found no remaining parity blocker

Pending CI/deployed validation

• Pact provider verification must run against the real broker; the local attempt was environment-inconclusive because localhost:9292 was unavailable
• Jenkins must validate image build, preview startup/readiness, smoke and functional behaviour
• normalized OpenAPI comparison and representative existing-schema Flyway validation remain explicit pre-merge checks

Temporary compatibility points

• remove the AM exclusion/mixin when a compatible AM client is published
• remove the Feign converter ordering workaround once the matching OpenFeign fix is available
• replace the BEFTA prerelease/Jackson 2 AAT pin when a compatible final BEFTA release is available

Does this PR introduce a breaking change?

[ ] Yes
[x] No