Fix pure annotations (#605)

Coming from https://github.com/ramsey/uuid/pull/603, this is an attempt
to fix the errors raised by the current phpstan settings.

I went through each of the errors raised by phpstan with the following
approach.

- If a method is part of an `@immutable` class, we can consider it pure,
  assuming it only affects internal variables.
- If a potentially pure method is calling a class's method that is only
  swapped during testing (and not during normal usage), then we can
  consider the calling method pure.
- If a class is marked deprecated, don't bother with attempting to mark
  it pure or immutable.

src/UuidFactory.php

@@ -396,6 +396,7 @@ class UuidFactory implements UuidFactoryInterface
      */
     public function uuid8(string $bytes): UuidInterface
     {
+        /** @phpstan-ignore possiblyImpure.methodCall */
         return $this->uuidFromBytesAndVersion($bytes, Uuid::UUID_TYPE_CUSTOM);
     }
 
@@ -439,6 +440,7 @@ class UuidFactory implements UuidFactoryInterface
 
         $bytes = $this->nameGenerator->generate($ns, $name, $hashAlgorithm);
 
+        /** @phpstan-ignore possiblyImpure.methodCall */
         return $this->uuidFromBytesAndVersion(substr($bytes, 0, 16), $version);
     }