chore: fix php7.4 compability

UPGRADING.md

@@ -160,7 +160,7 @@ $client->payments->page();
 Two new collection methods were added which can be used to simplify interacting with collection resources.
 
 - `contains(callable $callback): bool`
-- `filter(callable $callback): static`
+- `filter(callable $callback): self`
 
 ### Testmode is automatically removed..
 ..If an API key is used as authentication.

src/Factories/Factory.php

@@ -19,7 +19,7 @@ public function __construct($data = null)
         }
     }
 
-    public static function new(...$args): static
+    public static function new(...$args): self
     {
         return new static(...$args);
     }

src/Factories/RequestFactory.php

@@ -10,14 +10,14 @@ abstract class RequestFactory extends Factory
 
     private array $query = [];
 
-    public function withPayload(array $payload): static
+    public function withPayload(array $payload): self
     {
         $this->payload = $payload;
 
         return $this;
     }
 
-    public function withQuery(array $query): static
+    public function withQuery(array $query): self
     {
         $this->query = $query;
 

src/Factories/SortablePaginatedQueryFactory.php

@@ -6,7 +6,7 @@
 
 class SortablePaginatedQueryFactory extends RequestFactory
 {
-    public static function new(...$args): static
+    public static function new(...$args): self
     {
         return new static(...$args);
     }