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/MacroTest.php

@@ -117,7 +117,7 @@ class MacroTest extends AbstractTestCaseWithOldNow
 
         /** @var mixed $date */
         $date = Carbon::now();
-        $date->nonExistingMacro();
+        $result = $date->nonExistingMacro();
     }
 
     public function testTraitMixin()