Upgrade Spring Boot to 2.6.x

**Is your feature request related to a problem? Please describe.**
As Spring Boot 2.4.x is [marked](https://spring.io/projects/spring-boot#support) as `end of support`, it's better we upgrade it to 2.6.x for the 2.0.0 major release.

![image](https://user-images.githubusercontent.com/837658/160513095-8b98a436-323b-44d0-8a81-5b9305af3e8c.png)
