[php] PHP 7.3 end of life

[ybelenko]

PHP 7.3 reached end of life at 6 Dec 2020

End of life - A release that is no longer supported. Users of this release should upgrade as soon as possible, as they may be exposed to unpatched security vulnerabilities.

PHP: Supported Versions:

• 7.3 until 6 Dec 2021
• 7.4 until 28 Nov 2022
• 8.0 until 26 Nov 2023

It seems that we need to change minimum PHP version to 7.4 in all php generators.

Current Requirements:

Generator	PHP version
php client	^7.4 || ^8.0
Laravel server	>= 7.2.5
Lumen server	>= 7.2.5
Slim4 server	^7.4 || ^8.0
Symfony server	>= 7.1.3
Ze-ph server	^ 7.3 || ^8.0

cc @jebentier @dkarlovi @mandrean @jfastnacht @ackintosh @renepardon
cc @Articus as active PHP contributor

[Articus]

List of affected generators is a bit outdated - ze-ph was renamed to php-mezzio-ph (Mezzio Path Handler server) and there is new php-dt client (Data Transfer client) :) I'll update both in a week or two.

[ybelenko]

Closed in favour of #14152