Spring Revise Checkstyle rules to prohibit use of assertions other than AssertJ

Commit 9b3f4567eefcf1615b321a17b4dcbc1d71db4f83 should have failed the build since it imports and uses org.junit.jupiter.api.Assertions.

I've addressed that in 31a07e6fbf6c647352821d30540b9275e0ebf1a3, but we need to revise our Checkstyle rules to prevent accidental usage of other assertion APIs.