chores: roll back to spring boot 2.7.18 as it is the last version supporting java 8

Signed-off-by: Michele <lars.michele@tu-dortmund.de>

Author: lyca

spring/pom.xml

@@ -32,7 +32,7 @@
 
     <properties>
         <module-name>io.cloudevents.spring</module-name>
-        <spring-boot.version>3.1.8</spring-boot.version>
+        <spring-boot.version>2.7.18</spring-boot.version>
     </properties>
 
     <dependencyManagement>