Deprecate mysqli driver_version property

The driver version hasn't been updated in 13 years, so this number
is essentially meaningless. Check PHP_VERSION_ID instead.

UPGRADING

@@ -212,6 +212,11 @@ PHP 8.1 UPGRADE NOTES
 4. Deprecated Functionality
 ========================================
 
+- MySQLi:
+  . The mysqli_driver::$driver_version property has been deprecated. The driver
+    version is meaningless as it hasn't been updated in more than a decade. Use
+    PHP_VERSION_ID instead.
+
 ========================================
 5. Changed Functions
 ========================================

ext/mysqli/mysqli_driver.c

@@ -78,6 +78,7 @@ static int driver_client_info_read(mysqli_object *obj, zval *retval, bool quiet)
 /* {{{ property driver_driver_version_read */
 static int driver_driver_version_read(mysqli_object *obj, zval *retval, bool quiet)
 {
+	zend_error(E_DEPRECATED, "The driver_version property is deprecated");
 	ZVAL_LONG(retval, MYSQLI_VERSION_ID);
 	return SUCCESS;
 }

ext/mysqli/tests/mysqli_class_mysqli_driver_interface.phpt

@@ -54,7 +54,6 @@ printf("driver->client_info = '%s'\n", $driver->client_info);
 assert(mysqli_get_client_version() === $driver->client_version);
 printf("driver->client_version = '%s'\n", $driver->client_version);
 
-assert($driver->driver_version > 0);
 printf("driver->driver_version = '%s'\n", $driver->driver_version);
 
 assert(in_array($driver->report_mode, [
@@ -103,6 +102,8 @@ report_mode
 Magic, magic properties:
 driver->client_info = '%s'
 driver->client_version = '%d'
+
+Deprecated: The driver_version property is deprecated in %s on line %d
 driver->driver_version = '%d'
 driver->report_mode = '%d'
 driver->reconnect = ''

ext/mysqli/tests/mysqli_driver.phpt

@@ -27,11 +27,6 @@ if (($tmp = $driver->client_version) !== $client_version) {
     gettype($tmp), $tmp);
 }
 
-if (!is_int($tmp = $driver->driver_version) || (0 == $tmp)) {
-    printf("[004] Expecting int/any, got %s/%s\n",
-    gettype($tmp), $tmp);
-}
-
 $all_modes = [
     MYSQLI_REPORT_ALL,
     MYSQLI_REPORT_STRICT,