Find the reference to Spring Framework in the build.gradle file in the source for that Spring Boot version in GitHub.
For example, from the build.gradle for Spring Boot 2.6.6:
library("Spring Framework", "5.3.18")another way
Here is a quick way to find which version of spring-core is used.
Google “spring-boot-starter-test”.
Click on the first link that comes back - spring-boot-starter-test on mvnrepository
Click in the version of interest, eg 2.6.6
Find the spring-core dependency: 5.3.18