docs(travis): Document that PHP 7.4 testing is blocked on Drupal core compatibility

Author: klausi

.travis.yml

@@ -34,9 +34,8 @@ matrix:
         - DRUPAL_CORE=8.8.x
         # Only run code coverage on the latest php and drupal versions.
         - WITH_PHPDBG_COVERAGE=true
-    # PHP 7.4 testing is allowed to fail because the GD extension is not
-    # packaged on Travis CI yet.
-    # See https://travis-ci.community/t/some-extensions-are-missing-in-php-7-4-0-zip-gmp-sodium/6320/9
+    # PHP 7.4 testing is allowed to fail because Drupal core is not compatible
+    # yet. See https://www.drupal.org/project/drupal/issues/3086374
     - php: 7.4
 
 mysql: