Removed DateTimeImmutable::setTimestamp() from the CallMap: fully covered by stub

Also simplified the return type from `static|false` to `static`, since
the method throws at all times, on failure.

On PHP 7.x, it could only fail if an invalid type was passed in, which is
not really valid anyway, from a type perspective.

Ref (PHP 8.1.x): 32d55f7422/ext/date/php_date.c (L3353-L3369)
Ref (PHP 7.0.33): bf574c2b67/ext/date/php_date.c (L3596-L3612)

dictionaries/CallMap.php

@@ -1798,7 +1798,6 @@ return [
 'DateTimeImmutable::createFromInterface' => ['self', 'object' => 'DateTimeInterface'],
 'DateTimeImmutable::createFromMutable' => ['static', 'datetime'=>'DateTime'],
 'DateTimeImmutable::getLastErrors' => ['array{warning_count:int,warnings:array<int,string>,error_count:int,errors:array<int,string>}'],
-'DateTimeImmutable::setTimestamp' => ['static|false', 'unixtimestamp'=>'int'],
 'DateTimeInterface::diff' => ['DateInterval', 'datetime2'=>'DateTimeInterface', 'absolute='=>'bool'],
 'DateTimeInterface::format' => ['string', 'format'=>'string'],
 'DateTimeInterface::getOffset' => ['int'],

dictionaries/CallMap_historical.php

@@ -1059,7 +1059,6 @@ return [
     'DateTimeImmutable::__wakeup' => ['void'],
     'DateTimeImmutable::createFromMutable' => ['static', 'datetime'=>'DateTime'],
     'DateTimeImmutable::getLastErrors' => ['array{warning_count:int,warnings:array<int,string>,error_count:int,errors:array<int,string>}'],
-    'DateTimeImmutable::setTimestamp' => ['static|false', 'unixtimestamp'=>'int'],
     'DateTimeInterface::diff' => ['DateInterval', 'datetime2'=>'DateTimeInterface', 'absolute='=>'bool'],
     'DateTimeInterface::format' => ['string', 'format'=>'string'],
     'DateTimeInterface::getOffset' => ['int'],

stubs/CoreImmutableClasses.phpstub

@@ -91,7 +91,7 @@ class DateTimeImmutable implements DateTimeInterface
 
     /**
      * @psalm-mutation-free
-     * @return static|false
+     * @return static
      */
     public function setTimestamp(int $unixtimestamp) {}