Issue #3311472 Set minimum stability for form

Closes #3311472

src/Validator/VersionPolicy/TargetVersionNotPreRelease.php

+<?php
+
+declare(strict_types = 1);
+
+namespace Drupal\automatic_updates\Validator\VersionPolicy;
+
+use Composer\Semver\VersionParser;
+use Drupal\Core\StringTranslation\StringTranslationTrait;
+
+/**
+ * A policy rule requiring the target version to be a RC or higher.
+ *
+ * @internal
+ *   This is an internal part of Automatic Updates' version policy for
+ *   Drupal core. It may be changed or removed at any time without warning.
+ *   External code should not interact with this class.
+ */
+final class TargetVersionNotPreRelease {
+
+  use StringTranslationTrait;
+
+  /**
+   * Checks that the target version of Drupal is a stable release.
+   *
+   * @param string $installed_version
+   *   The installed version of Drupal.
+   * @param string|null $target_version
+   *   The target version of Drupal, or NULL if not known.
+   *
+   * @return \Drupal\Core\StringTranslation\TranslatableMarkup[]
+   *   The error messages, if any.
+   */
+  public function validate(string $installed_version, ?string $target_version): array {
+    if (!in_array(VersionParser::parseStability($target_version), ['stable', 'RC'], TRUE)) {
+      return [
+        $this->t('Drupal cannot be updated to the recommended version, @target_version, because it is not a stable version.', [
+          '@target_version' => $target_version,
+        ]),
+      ];
+    }
+    return [];
+  }
+
+}