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

buildSrc/src/main/java/org/springframework/boot/build/JavaConventions.java

@@ -190,9 +190,7 @@ class JavaConventions {
 		if (isPredictiveTestSelectionEnabled()) {
 			PredictiveTestSelectionExtension predictiveTestSelection = test.getExtensions()
 				.getByType(PredictiveTestSelectionExtension.class);
-			predictiveTestSelection.getEnabled().set(true);
-			predictiveTestSelection.mustRun((mustRun) -> mustRun.getIncludeClasses()
-				.add("org.springframework.boot.actuate.autoconfigure.endpoint.web.documentation.*"));
+			predictiveTestSelection.getEnabled().convention(true);
 		}
 	}