Add #[NoDiscard] to CarbonImmutable modify(), next(), previous() and __call()

On PHP 8.5, DateTimeImmutable::modify() warns when its result is
discarded, but CarbonImmutable overrides it and lost the attribute.
Override modify(), next(), previous() and __call() in CarbonImmutable
with #[NoDiscard] so a discarded result warns again. The attribute is
ignored on older PHP versions, and mutable Carbon is unchanged.

Existing immutable tests that discarded results now keep them; in
IsTest::testIsSameMonth this was a real slip, the loop never moved $dt.

Fixes #3377

tests/CarbonImmutable/GenericMacroTest.php

@@ -55,7 +55,7 @@ class GenericMacroTest extends AbstractTestCaseWithOldNow
         $message = null;
 
         try {
-            $now->barBiz();
+            $result = $now->barBiz();
         } catch (BadMethodCallException $exception) {
             $message = $exception->getMessage();
         }