minor #50297 Remove unnecessary usages of DateTime (nicolas-grekas)

This PR was merged into the 6.4 branch.

Discussion
----------

Remove unnecessary usages of DateTime

| Q             | A
| ------------- | ---
| Branch?       | 6.3
| Bug fix?      | no
| New feature?  | no
| Deprecations? | no
| Tickets       | Part of #47580
| License       | MIT
| Doc PR        | -

Together with #50290, this PR removes `DateTime` everywhere possible.

What remains is:
- test cases that test both DateTime and DateTimeImmutable - legit to keep
- date/time form-types and transformers in the Form component - we'd need a separate effort for them, related to #50295
- `PersistentTokenInterface::getLastUsed(): \DateTime` - separate effort also

Commits
-------

8b08294b63 Remove unnecessary usages of DateTime

Author: fabpot

Tests/InlineTest.php

@@ -574,7 +574,7 @@ public function testParseTimestampAsUnixTimestampByDefault(string $yaml, int $ye
      */
     public function testParseTimestampAsDateTimeObject(string $yaml, int $year, int $month, int $day, int $hour, int $minute, int $second, int $microsecond, string $timezone)
     {
-        $expected = (new \DateTime($yaml))
+        $expected = (new \DateTimeImmutable($yaml))
             ->setTimeZone(new \DateTimeZone('UTC'))
             ->setDate($year, $month, $day)
             ->setTime($hour, $minute, $second, $microsecond);
@@ -599,7 +599,7 @@ public static function getTimestampTests(): array
      */
     public function testParseNestedTimestampListAsDateTimeObject(string $yaml, int $year, int $month, int $day, int $hour, int $minute, int $second, int $microsecond)
     {
-        $expected = (new \DateTime($yaml))
+        $expected = (new \DateTimeImmutable($yaml))
             ->setTimeZone(new \DateTimeZone('UTC'))
             ->setDate($year, $month, $day)
             ->setTime($hour, $minute, $second, $microsecond);