feature: upgrade Spring Boot 2.0.2 -> 4.1.1 (Java 17) incrementally - #79

Summary

Upgrades the app from Spring Boot 2.0.2.RELEASE / Java 8 (what master actually had — not 2.7.18) to Spring Boot 4.1.1 / Java 17, one minor version at a time with one commit per step. At every step mvn clean verify passed and the jar was started and smoke-tested (/topic, /topic/java, /?name=Devin, banner version checked).

Upgrade path (one commit each)

2.0.2.RELEASE → 2.1.18.RELEASE → 2.2.13.RELEASE → 2.3.12.RELEASE → 2.4.13 → 2.5.15 → 2.6.15 → 2.7.18 → 3.0.13 → 3.1.12 → 3.2.12 → 3.3.13 → 3.4.13 → 3.5.16 → 4.0.8 → 4.1.1

Breaking changes handled

Not changed

• build.gradle (still references Spring Boot 2.0.2 / Gradle compile config) was left as-is; the Maven build is the one used by CI/blueprint. Happy to update in a follow-up if wanted.
• Tracked target/ build artifacts were not re-committed.
• No tests exist under src/test, so nothing to run there.

Verification (Spring Boot 4.1.1, Java 17)

Full log in the attached springboot-4.1.1-evidence.txt (session attachment). Excerpt:

 :: Spring Boot ::                (v4.1.1)
INFO ... hello.Application : Started Application in 1.493 seconds

$ curl -s -i localhost:8080/topic                 -> 200 [{"id":"spring",...},{"id":"java",...},{"id":"javascript",...}]
$ curl -s -i localhost:8080/topic/java            -> 200 {"id":"java","subjectName":"Core Java",...}
$ curl -s -i "localhost:8080/?name=Devin"         -> 200 {"id":1,"content":"Hello, Devin!"}
$ curl -X POST   .../topic  {"id":"kotlin",...}   -> 200 ; GET /topic/kotlin -> 200 {"id":"kotlin","subjectName":"Kotlin",...}
$ curl -X PUT    .../topic/kotlin {...Updated}    -> 200 ; GET /topic/kotlin -> {"subjectName":"Kotlin Updated",...}
$ curl -X DELETE .../topic/kotlin                 -> 200 ; GET /topic -> back to 3 original topics
$ curl .../topic/sort, /topic/minimum/length/4, /topic/string/operation, /datetime -> 200
$ curl -s -i localhost:8080/actuator/health       -> 200 {"groups":["liveness","readiness"],"status":"UP"}

Devin-Org: engineering

Link to Devin session: https://app.devin.ai/sessions/6e5668cfa0694943861c66ec8c141646
Open in Devin Desktop: https://app.devin.ai/desktop/session/6e5668cfa0694943861c66ec8c141646?variant=devin
Requested by: @clivingston-cognition