Upgrade to AssertJ 3.27.3

[wilkinsona]

Upgrade to AssertJ 3.27.3.

[wilkinsona]

This causes a problem with Kotlin:

> Task :spring-boot-project:spring-boot-docs:compileKotlin
e: file:///Users/awilkinson/dev/spring-projects/spring-boot/main/spring-boot-project/spring-boot-docs/src/main/kotlin/org/springframework/boot/docs/testing/springbootapplications/autoconfiguredwebservices/client/MyWebServiceClientTests.kt:36:42 Overload resolution ambiguity: 
public open fun <T : Any!> extracting(extractor: Function<in Response!, TypeVariable(T)!>!): AbstractObjectAssert<*, TypeVariable(T)!>! defined in org.assertj.core.api.ObjectAssert
public final fun <T : Any!> extracting(vararg extractors: Function<in Response!, out TypeVariable(T)!>!): AbstractListAssert<*, (MutableList<out TypeVariable(T)!>..List<TypeVariable(T)!>?), TypeVariable(T)!, ObjectAssert<TypeVariable(T)!>!>! defined in org.assertj.core.api.ObjectAssert

I believe it's an unexpected side-effect of assertj/assertj#3673. I've opened assertj/assertj#3728.