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/SettersTest.php

@@ -52,7 +52,7 @@ class SettersTest extends AbstractTestCase
         ));
 
         $d = Carbon::now();
-        $d->setHours(Month::February);
+        $d = $d->setHours(Month::February);
     }
 
     public function testMonthEnum()
@@ -74,7 +74,7 @@ class SettersTest extends AbstractTestCase
         ));
 
         $d = Carbon::parse('2023-10-25 21:14:51');
-        $d->setMonth(2.5);
+        $d = $d->setMonth(2.5);
     }
 
     public function testMonthFloatPassWithZeroDecimalPart()