Decouple Maven plugin's integration tests from spring-boot-dependencies

[wilkinsona]

The changes made for #23936 meant that any change to the spring-boot-dependencies bom results in all of our Maven plugin's integration tests being run as the effective bom is an input of the intTest task. The integration tests are quite slow (they took 23 minutes in this CI build for example) and I'm not sure that the benefits of the changes for #23936 are worth the current cost.

[wilkinsona]

We think we might be able to pre-process the effective bom in spring-boot-maven-plugin to extract just the versions that we need and then use those in the plugin's tests.