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/ModifyTest.php

@@ -249,6 +249,6 @@ class ModifyTest extends AbstractTestCase
             '$anchorDay is not compatible with overflow',
         ));
 
-        Carbon::parse('2023-02-28')->addYearsWithOverflow(2, anchorDay: 29);
+        $date = Carbon::parse('2023-02-28')->addYearsWithOverflow(2, anchorDay: 29);
     }
 }