[Build] Require reason for usesDefaultDistribution (#124707)

This makes using usesDefaultDistribution in our test setup for explicit by requiring a reason why it's needed.
This is helpful as part of revisiting the need for all those usages in our code base.

x-pack/plugin/downsample/qa/mixed-cluster/build.gradle

@@ -31,7 +31,7 @@ def supportedVersion = bwcVersion -> {
 buildParams.bwcVersions.withWireCompatible(supportedVersion) { bwcVersion, baseName ->
 
   def yamlRestTest = tasks.register("v${bwcVersion}#yamlRestTest", StandaloneRestIntegTestTask) {
-    usesDefaultDistribution()
+    usesDefaultDistribution("BWC tests require full distribution for now")
     usesBwcDistribution(bwcVersion)
     systemProperty("tests.old_cluster_version", bwcVersion)
     testClassesDirs = sourceSets.yamlRestTest.output.classesDirs

x-pack/plugin/downsample/qa/rest/build.gradle

@@ -25,10 +25,10 @@ artifacts {
 }
 
 tasks.named('yamlRestTest') {
-  usesDefaultDistribution()
+  usesDefaultDistribution("to be triaged")
 }
 tasks.named('yamlRestCompatTest') {
-  usesDefaultDistribution()
+  usesDefaultDistribution("to be triaged")
 }
 if (buildParams.inFipsJvm){
   // This test cluster is using a BASIC license and FIPS 140 mode is not supported in BASIC