EZP-31542: Added PHP 7.4 support (eZ Platform 3.0)

[adamwojs]

Question	Answer
JIRA issue	EZP-31542
Type	improvement
BC breaks	no
Tests pass	yes
Doc needed	no

The most significant changes from our perspective:

Trying to use values of type null, bool, int, float or resource as an array (such as $null["key"]) will now generate a notice. This does not affect array accesses performed by list(). RFC: https://wiki.php.net/rfc/notice-for-non-valid-array-container

Passing invalid characters to ''base_convert()'', ''bindec()'', ''octdec()'' and ''hexdec()'' will now generate a deprecation notice. The result will still be computed as if the invalid characters did not exist. Leading and trailing whitespace, as well as prefixes of type 0x (depending on base) continue to be allowed.

Throwing exceptions from __toString() is now permitted. Previously this resulted in a fatal error. Existing recoverable fatals in string conversions have been converted to Error exceptions. RFC: https://wiki.php.net/rfc/tostring_exceptions

Full upgrade notes: https://github.com/php/php-src/blob/PHP-7.4.5/UPGRADING

Checklist:

• PR description is updated.
• Tests are implemented.
• Added code follows Coding Standards (use $ composer fix-cs).
• PR is ready for a review.

[alongosz]

Needs to be a merge up from ezsystems/ezpublish-kernel#3017
Nitpick (not sure to which PR applies):