RFC: Deprecate date constant RFC7231

[jorgsowa]

This PR deprecates constants DATE_RFC7231 and DateTimeInterface::RFC7231.

Because the implementation of the constant contains the timezone GMT designation, the format of date will always show the GMT timezone, despite the real timezone of the instance of DateTimeInterface. This may lead to unwanted results.

The implementation of this constant was a mistake, as this was described as bug. This wasn't bug, this was missing functionality: #2450

Example of wrong display of datetime:

$date = new \DateTime();
$date->setTimeZone(new DateTimeZone('GMT')); //Wed, 20 Dec 2023 23:28:11 GMT
echo $date->format(DATE_RFC7231)."\n";
$date->setTimeZone(new DateTimeZone('Europe/Tallinn')); //Thu, 21 Dec 2023 01:28:20 GMT
echo $date->format(DATE_RFC7231);

https://3v4l.org/5Oh7Z

I also suggest adding this to the collective RFC https://wiki.php.net/rfc/deprecations_php_8_4 What do you think @Girgias?

[Girgias]

Adding this to the mass deprecation makes sense, but I would want to wait for @derickr's opinion, but he's on holidays for 2 (3?) weeks. Please ping us again in January if we forget this :/

[derickr]

I am ok with this being added to the 8.4 deprecations. I wonder whether we can also not clean up some of the tests to not even use this constant in there, as there are now so many deprecation warnings in the test. I suggest we only keep the tests for these constants for DateTimeInterface_constants. No biggy though, but just as well we can clean up something here.

[medabkari]

Is this still part of the deprecation list for 8.4?

[jorgsowa]

Unfortunately no. I completely forgot about this PR and I didn't add this for the 8.4 deprecations collection. I will add for the next one.

[jorgsowa]

Added to https://wiki.php.net/rfc/deprecations_php_8_5