Draft: Resolve "Upgrade spring-boot dependency (2.3.12 instead of 2.2.11)"

Closes #601 (closed)

Upgrade spring-boot dependency (2.3.12 instead of 2.2.11)

see:

• https://spring.io/projects/spring-boot#learn
• https://spring.io/blog/2020/12/10/spring-boot-2-2-12-available-now
• https://spring.io/blog/2021/01/14/spring-boot-2-2-13-available-now
• https://github.com/spring-projects/spring-boot/wiki/Spring-Boot-2.3-Release-Notes
• https://spring.io/blog/2020/05/15/spring-boot-2-3-0-available-now
• https://spring.io/blog/2020/06/12/spring-boot-2-3-1-available-now
• https://spring.io/blog/2020/07/24/spring-boot-2-3-2-available-now
• https://spring.io/blog/2020/09/17/spring-boot-2-3-4-available-now
• https://spring.io/blog/2020/10/29/spring-boot-2-3-5-available-now
• https://spring.io/blog/2020/11/13/spring-boot-2-3-6-available-now
• https://spring.io/blog/2020/12/11/spring-boot-2-3-7-available-now
• https://spring.io/blog/2021/01/14/spring-boot-2-3-8-available-now
• https://spring.io/blog/2021/02/18/spring-boot-2-3-9-is-now-available
• https://spring.io/blog/2021/04/15/spring-boot-2-3-10-is-now-available
• https://spring.io/blog/2021/05/20/spring-boot-2-3-11-available-now
• https://spring.io/blog/2021/06/10/spring-boot-2-3-12-release-available-now