What are the key breaking changes removed in Spring Boot 4 that were deprecated in Boot 3?

Spring Boot 4 follows a strict deprecation policy: everything deprecated in Spring Boot 3.x is removed in 4.0 with no fallback. This is why the recommended migration path is to first upgrade to Boot 3.5 and fix all deprecation warnings before touching Boot 4.

# WRONG (Boot 3 style, removed in Boot 4):
# spring.jackson2.serialization.write-dates-as-timestamps=false

# CORRECT (Boot 4):
spring.jackson.serialization.write-dates-as-timestamps=false

// Migration script check: find deprecated patterns
// Run OpenRewrite to automate many changes:
// mvn rewrite:run \
//   -Drewrite.activeRecipes=\
//     org.openrewrite.java.spring.boot3.UpgradeSpringBoot_3_5,\
//     org.openrewrite.java.spring.boot4.UpgradeSpringBoot_4_0

// RestTemplate to RestClient migration:
// Spring Boot 3 (discouraged):
RestTemplate rt = new RestTemplate();
String result = rt.getForObject("http://api/data", String.class);

// Spring Boot 4 (recommended):
RestClient client = RestClient.create();
String result = client
    .get()
    .uri("http://api/data")
    .retrieve()
    .body(String.class);

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