Build: Add warning when bwc tests are disabled (#28585)

The bwc tests can be disabled in order to facilitate commits necessary
to older branches to maintain backcompat. A check already exists to
ensure this flag is not left disabled too long (once a day by the
branchConsistency check). However, it can be surprising if you try
running bwc tests explicitly and they look like nothing is happening.
This commit adds a warning during configuration to ensure it is clear
the bwc tests are disabled and enforces a link to a PR which is in the
process of being backported.

build.gradle

@@ -144,13 +144,24 @@ task verifyVersions {
  * the enabled state of every bwc task. It should be set back to true
  * after the backport of the backcompat code is complete.
  */
-allprojects {
-  ext.bwc_tests_enabled = true
+final boolean bwc_tests_enabled = true
+final String bwc_tests_disabled_issue = "" /* place a PR link here when commiting bwc changes */
+if (bwc_tests_enabled == false) {
+  if (bwc_tests_disabled_issue.isEmpty()) {
+    throw new GradleException("bwc_tests_disabled_issue must be set when bwc_tests_enabled == false")
+  }
+  println "========================= WARNING ========================="
+  println "         Backwards compatibility tests are disabled!"
+  println "See ${bwc_tests_disabled_issue}"
+  println "==========================================================="
+}
+subprojects {
+  ext.bwc_tests_enabled = bwc_tests_enabled
 }
 
 task verifyBwcTestsEnabled {
   doLast {
-    if (project.bwc_tests_enabled == false) {
+    if (bwc_tests_enabled == false) {
       throw new GradleException('Bwc tests are disabled. They must be re-enabled after completing backcompat behavior backporting.')
     }
   }