Bump org.springframework.boot:spring-boot-starter-parent in /server

Bumps [org.springframework.boot:spring-boot-starter-parent](https://github.com/spring-projects/spring-boot) from 3.5.6 to 4.0.0.
- [Release notes](https://github.com/spring-projects/spring-boot/releases)
- [Commits](spring-projects/spring-boot@v3.5.6...v4.0.0)

---
updated-dependencies:
- dependency-name: org.springframework.boot:spring-boot-starter-parent
  dependency-version: 4.0.0
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

server/ssh-key-signer-server/pom.xml

@@ -20,7 +20,7 @@
     <parent>
         <groupId>org.springframework.boot</groupId>
         <artifactId>spring-boot-starter-parent</artifactId>
-        <version>3.5.6</version>
+        <version>4.0.0</version>
         <relativePath />
     </parent>