Remove support for Spring Session Data MongoDB (#47662)

[likhith5697]

This PR removes built-in support for Spring Session Data MongoDB.

Summary of Changes

• Deleted module:spring-boot-session-data-mongodb and related starter/smoke-test modules.
• Removed references from settings.gradle and submodule build files.
• Updated spring-boot-dependencies to exclude MongoDB session artifacts.
• Cleaned up metadata and documentation mentions.

Rationale
MongoDB session support is now maintained by the MongoDB team:
https://github.com/mongodb/spring-session-mongodb

Resolves: #47662

[philwebb]

Thanks, but this issue was already assigned and in progress.

[likhith5697]

Thanks, but this issue was already assigned and in progress.

Thanks @philwebb! Appreciate the clarification — this was a great learning experience for contributing to Spring Boot.