chore: use same spring version (#29)

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## Description
<!-- Provide a detailed description of the changes -->

Updates the example stub to use Spring Boot 3.2.4. Also extracts the
Spring Boot and Dependency Manager plugin versions to share between
projects.

## Review Checklist
- [x] I have clicked on ["allow edits by
maintainers"](https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/allowing-changes-to-a-pull-request-branch-created-from-a-fork).
- [ ] I have added documentation for new/changed functionality in this
PR or in a PR to [openfga.dev](https://github.com/openfga/openfga.dev)
[Provide a link to any relevant PRs in the references section above]
- [x] The correct base branch is being used, if not `main`
- [ ] I have added tests to validate that the change in functionality is
working as expected

examples/servlet/build.gradle

@@ -1,7 +1,7 @@
 plugins {
     id 'java'
-    id 'org.springframework.boot' version '3.2.2'
-    id 'io.spring.dependency-management' version '1.1.4'
+    id 'org.springframework.boot' version "${springBootVersion}"
+    id 'io.spring.dependency-management' version "${springDependencyManagementVersion}"
 }
 
 dependencies {

gradle.properties

@@ -0,0 +1,2 @@
+springBootVersion=3.2.4
+springDependencyManagementVersion=1.1.4

openfga-spring-boot-starter/build.gradle

@@ -1,7 +1,7 @@
 plugins {
     id 'java-library'
-    id 'org.springframework.boot' version '3.2.4' apply false
-    id 'io.spring.dependency-management' version '1.1.4'
+    id 'org.springframework.boot' version "${springBootVersion}" apply false
+    id 'io.spring.dependency-management' version "${springDependencyManagementVersion}"
     id 'maven-publish'
 }