chore(deps): update Spring Boot to 4.1.0 and Gradle wrapper to 9.6.1

[dex-the-ai]

Summary

Recreates #89 under the dex-the-ai account, as requested in #89 (comment). The diff is identical to #89's head (git diff pr89 is empty).

• Spring Boot Gradle plugin 4.0.6 → 4.1.0
• Gradle wrapper 9.5.0 → 9.6.1 (regenerated with ./gradlew wrapper --gradle-version 9.6.1 --distribution-type bin)
• ravsamhq/notify-slack-action v1 → v2. v1's Docker action fails in GitHub Actions with TypeError: 'type' object is not subscriptable. v2 runs on node20 and takes the same inputs.

Supersedes #89. Also covers Dependabot #86 (Spring Boot 4.1.0). Dependabot #91 proposes Gradle 9.7.0. That is newer than this PR's version and can land on top afterwards.

Validation (run 2026-09-29, against a Couchbase cluster with travel-sample)

Check	Command	Result
Diff matches #89	git diff pr89	No differences
Wrapper version	./gradlew --version	Gradle 9.6.1
Workflow YAML parse	python3 -c "import yaml; yaml.safe_load(open('.github/workflows/tests.yaml'))"	OK
Integration tests, JDK 21	./gradlew clean test --stacktrace	BUILD SUCCESSFUL in 1m 1s, 18 tests, 0 failures, 0 errors, 0 skipped
Integration tests, JDK 17	./gradlew clean test -Dorg.gradle.java.home=<jdk17>	BUILD SUCCESSFUL in 48s, 18 tests, 0 failures/errors
App boot	./gradlew bootRun	Spring Boot :: (v4.1.0), Started Application in 5.349 seconds
OpenAPI	curl -fsS localhost:8080/v3/api-docs	OAS 3.1.0, title Quickstart in Couchbase with Spring Data, version 2.0, 8 paths
Swagger UI	curl localhost:8080/swagger-ui/index.html	HTTP 200
Live API	curl -fsS localhost:8080/api/v1/airline/airline_10	{"id":"airline_10","type":"airline","name":"40-Mile Air","iata":"Q5","icao":"MLA","callsign":"MILE-AIR","country":"United States"}

Per-class test results (JDK 21):

AirlineIntegrationTest  7 tests, 0 failures, 0 errors
AirportIntegrationTest  6 tests, 0 failures, 0 errors
RouteIntegrationTest    5 tests, 0 failures, 0 errors
TOTAL                  18 tests, 0 failures, 0 errors, 0 skipped

Risk / rollback

These are minor and tooling updates. No build-script migration was needed. To roll back, revert this PR.

🤖 Generated with Claude Code