[core] Drop php 5.6 and php 7.0 support

[em92]

php 5.6 reached end of life on Dec 31, 2018
php 7.0 reached end of life on Jan 10, 2019
https://www.php.net/eol.php

Closes #863

Reason to drop support is use getenv() without "varname" parameter, that it ised in #2100

TODOs:

• update composer.lock
• change array syntax everywhere (array() -> [])

[em92]

change array syntax everywhere (array() -> [])

Removing from todo. Looks like long array syntax is introduced in php 5.4, while I was thinking it is introduced in 7.0

[em92]

Php 7.0 introduced "??". So it would be good to force use it with lints.
https://stackoverflow.com/questions/53610622/what-does-double-question-mark-operator-mean-in-php

[dvikan]

Soon, php 7.3 will not be getting anymore security updates[0].

In my opinion we should enforce a minimum of php 7.4 in order to avoid security problems alltogether.

It also makes the life of developers easier when they can use more recent language features such as the ?? operator and strong typing (also on return values).

[0] https://www.php.net/supported-versions.php

[em92]

Lets skip those new language features for now.

[em92]

In my opinion we should enforce a minimum of php 7.4 in order to avoid security problems alltogether.

Lets allow user to use EOL php on their risk.

Offtopic: In my fulltime paid work (Odoo related consultation) some customers are still running ERPs on Python 2 (reached EOL on Jan 1, 2020) or EOL versions of Python 3.