Minimum requirements PHP7 and WP5

[herrvigg]

The minimum requirements for QTX are PHP 5.4 and WP 4.8, it has not changed for a while.
According to the WP stats more than ~90% of the websites are with PHP 7.0+ and WP 5.0+.
I plan to uplift the minimum requirements as such in the next major release. This will enable better PHP code by dropping the legacy.

I would rather go for minimum PHP 7.3+ as it's the oldest version still supported. But we don't strictly need the new features in QTX so we can be more conservative for the next move.

[herrvigg]

The required versions are now bumped to PHP 7.0 and WP 5.0.
This is very conservative so it will happen in next minor release 3.15.0.
The WordPress websites are lagging behind quite a lot but I'd like to upgrade PHP more, PHP7.4 has already reached end of life and PHP 8.0 is in security fixes only.

[herrvigg]

I've already bumped the required PHP version from 7.0 to 7.1 since there's only a small 2.3% loss difference. We may even go to PHP7.2 losing only 1.6% more.

Now adding PHP type declarations that makes the code clearer and catches some bugs due to wrong calls. PHP7.1 is interesting for nullable args and void return values.

[herrvigg]

See #1314 for PHP type declarations up to PHP7.1.

[herrvigg]

Released in 3.15.0.