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

README.md

@@ -57,7 +57,7 @@ $email = (new TemplatedEmail())
     // ...
     ->htmlTemplate('emails/signup.html.twig')
     ->context([
-        'expiration_date' => new \DateTime('+7 days'),
+        'expiration_date' => new \DateTimeImmutable('+7 days'),
         'username' => 'foo',
     ])
 ;