Update Spring Boot to the latest version

[robertmcnees]

Update the guide to use the most recent Spring Boot version.

Files that require changes are:

initial/build.gradle
initial/pom.xml
complete/build.gradle
complete/pom.xml

[zhangwt-cn]

Hello, which version do you need to upgrade to? I will be happy to do it.

[robertmcnees]

Hi @zhangwt-cn . Thank you for the interest.

You can find the latest stable Spring Boot version by finding the CURRENT tag on this page. Additionally, you could go to start.spring.io and find the latest version that is not a SNAPSHOT or a milestone (M).

As of this moment the latest version of Spring Boot is 3.1.4.

[zhangwt-cn]

Hi @robertmcnees I have submitted the modification, please feel free to correct me if there are any questions. #49