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/CarbonPeriod/GettersTest.php

@@ -90,7 +90,7 @@ class GettersTest extends AbstractTestCase
     {
         $period = CarbonPeriodFactory::withStartIntervalEnd(static::$periodClass);
 
-        $period->getStartDate()->subDays(3);
+        $date = $period->getStartDate()->subDays(3);
 
         $this->assertSame('2012-07-01', $period->getStartDate()->format('Y-m-d'));
     }
@@ -99,7 +99,7 @@ class GettersTest extends AbstractTestCase
     {
         $period = CarbonPeriodFactory::withStartIntervalEnd(static::$periodClass);
 
-        $period->getEndDate()->addDays(3);
+        $date = $period->getEndDate()->addDays(3);
 
         $this->assertSame('2012-07-15', $period->getEndDate()->format('Y-m-d'));
     }