fix(deps): bump spring-boot to 4.0.7 to resolve spring-webmvc/spring-webflux GHSA-mq64-j8f9-9gcj

This resolves Dependabot alerts #117 and #118 for spring-webmvc and
spring-webflux respectively. Both vulnerabilities (GHSA-mq64-j8f9-9gcj)
are in Spring Framework 7.0.7 and earlier. Spring Boot 4.0.7 is the
earliest 4.0.x patch release that manages Spring Framework 7.0.8+.

Co-Authored-By: Catarina Correia <catarina.correia@getyourguide.com>

Author: claude

gradle/libs.versions.toml

@@ -1,6 +1,6 @@
 [versions]
 java = "21"
-spring-boot = "4.0.6"
+spring-boot = "4.0.7"
 spring-dependency-management = "1.1.7"
 openapi-generator = "7.21.0"
 openapi-tools = "0.2.9"