configure bot to update spring-boot-dependencies and spring-boot-starter-parent together

`spring-boot-dependencies` is used in this project for published artifacts, and `spring-boot-starter-parent` is used in samples.
We should keep them in sync when upgrading. Otherwise may run into dependency issues with samples, see example in [b/416442436#comment13](https://b.corp.google.com/416442436#comment13)

Currently, seems that these artifacts are updated independently by dependabot:
https://github.com/GoogleCloudPlatform/spring-cloud-gcp/commit/7ab75a312bc2eccb5afca923870961b5dae5dd2b
https://github.com/GoogleCloudPlatform/spring-cloud-gcp/commit/ac2b0afe1d7629c04ffa100e44fd4161cde65e07