Previously, enforcedPlatform dependencies were using to pull in the
constraints defined in spring-boot-dependencies and
spring-boot-parent and applied them strictly so that the constrained
version had to be used. This worked as intended in Spring Boot's own
build but incorrectly enforced those same strict version requirements
on external consumers of Spring Boot's modules.

This commit reworks how Spring Boot defines its internal dependency
management so that platform dependencies are exposed to external
consumers while enforced platform dependencies are using internally.

See gh-19609