Use PHP's new class constant mechanism.

Let gen_stub.php define the constants for us, including deriving their
actual values from C defines.

As a side-effect we have to drop support for PHP < 7.2 as it does not
have interned strings.

redis_cluster.stub.php

@@ -7,6 +7,45 @@
  */
 
 class RedisCluster {
+    /**
+     *
+     * @var int
+     * @cvalue REDIS_OPT_FAILOVER
+     *
+     */
+    public const OPT_SLAVE_FAILOVER = UNKNOWN;
+
+    /**
+     *
+     * @var int
+     * @cvalue REDIS_FAILOVER_NONE
+     *
+     */
+    public const FAILOVER_NONE = UNKNOWN;
+
+    /**
+     *
+     * @var int
+     * @cvalue REDIS_FAILOVER_ERROR
+     *
+     */
+    public const FAILOVER_ERROR = UNKNOWN;
+
+    /**
+     *
+     * @var int
+     * @cvalue REDIS_FAILOVER_DISTRIBUTE
+     *
+     */
+    public const FAILOVER_DISTRIBUTE = UNKNOWN;
+
+    /**
+     *
+     * @var int
+     * @cvalue REDIS_FAILOVER_DISTRIBUTE_SLAVES
+     *
+     */
+    public const FAILOVER_DISTRIBUTE_SLAVES = UNKNOWN;
 
     public function __construct(string|null $name, array $seeds = NULL, int|float $timeout = 0, int|float $read_timeout = 0, bool $persistent = false, #[\SensitiveParameter] mixed $auth = NULL, array $context = NULL);