Bump spring-boot from 4.0.5 to 4.0.6 (CVE-2026-40976 / GHSA-8v8j-3hxp-93wr)

Agent-Logs-Url: https://github.com/getyourguide/openapi-validation-java/sessions/c9ccfb00-2084-4c14-87c3-e25e002397a4

Co-authored-by: catarina-correia <93409185+catarina-correia@users.noreply.github.com>

Author: Copilot

gradle/libs.versions.toml

@@ -1,6 +1,6 @@
 [versions]
 java = "21"
-spring-boot = "4.0.5"
+spring-boot = "4.0.6"
 spring-dependency-management = "1.1.7"
 openapi-generator = "7.21.0"
 openapi-tools = "0.2.9"