Improve bom checking

[wilkinsona]

#38228 has shown that we could improve our checking of the project's boms (spring-boot-dependencies and spring-boot-parent).

We could:

• Check that a prohibited version range isn't ineffective due to the library's current version being beyond its upper bound
• Check that a library's current version isn't prohibited

We could do this as part of the existing bomrCheck task and make the check task depend on this task so that it always runs.