Update the migration guide following the modularisation of spring-boot-test and spring-boot-test-autoconfigure

[wilkinsona]

Some things to document (there may be more):

• The new -test modules
• The other new modules that don't follow the -test pattern:
  • spring-boot-restdocs
  • spring-boot-resttestclient
  • spring-boot-webtestclient
• The new starter-…-test modules
• spring-boot-starter-test-classic
• org.springframework.boot.test.context.PropertyMapping.Skip as noticed by @fmbenhassine