Polish configuration related to predictive test selection

Most notably, this commit splits the tests that use Spring REST Docs
out into a separate task for which predictive test selection is
disabled. This allows it to be cached and use Gradle's built-in
up-to-date checking, thereby avoiding the generation of new snippets
and the need to then run the asciidoctor and asciidoctorPdf tasks.

It also updates spring-boot-smoke-test-junit-vintage to disable
predictive test selection so that we can continue to assert that
the some tests were executed.

See gh-35869

spring-boot-tests/spring-boot-smoke-tests/spring-boot-smoke-test-junit-vintage/build.gradle

@@ -23,4 +23,7 @@ test {
 			}
 	 	}
 	}
+	predictiveSelection {
+		enabled = false
+	}
 }