Updated search_api_solr vesion to fix the d9 test.

RoboFile.php

@@ -90,7 +90,13 @@ public function testFull(int $drupal_version = 9, string $site_name = NULL) {
     }
 
     // Composer require the corresponding modules, push to Pantheon and install the site.
-    $this->testRequireSolr($site_name, $constraint);
+    $version = "dev-4.x";
+    if ($drupal_version == 9) {
+      $version = "3.6";
+    }
+
+    $this->testRequireSolr($site_name, $version, $constraint);
+
     $this->testGitPush($site_name);
     $this->testConnectionGit($site_name, 'dev', 'sftp');
     $this->testSiteInstall($site_name);
@@ -447,14 +453,14 @@ public function testDowngradeToDrupal8(string $site_name) {
    * @param string $constraint
    *   The constraint to use for the search_api_pantheon module.
    */
-  public function testRequireSolr(string $site_name, string $constraint = '^8') {
+  public function testRequireSolr(string $site_name, string $version, string $constraint = '^8') {
     $site_folder = $this->getSiteFolder($site_name);
     chdir($site_folder);
     // Always test again latest version of search_api_solr.
     $this->taskExec('composer')
       ->args(
         'require',
-        'drupal/search_api_solr:dev-4.x',
+        'drupal/search_api_solr:' . $version,
       )
       ->run();
     $this->taskExec('composer')