Bump Spring Boot 3.4.4 -> 3.5.14

The 3.4.x line is no longer maintained on OSS support;
3.5.14 is the latest patch on the supported 3.5.x branch.

Verified locally on Temurin 24.0.2:
- ./gradlew clean build: SUCCESS
- ./gradlew test: AppTest.testRootPage PASSED
- ./gradlew checkstyleMain: no issues
- bootRun: app responds 200 on GET / with the same body

Author: le0nus

build.gradle.kts

@@ -4,7 +4,7 @@ import org.gradle.api.tasks.testing.logging.TestLogEvent
 plugins {
   jacoco
   checkstyle
-  id("org.springframework.boot") version "3.4.4"
+  id("org.springframework.boot") version "3.5.14"
   id("io.spring.dependency-management") version "1.1.7"
   id("application")
 }