Composer: bump PHP requirement to 7.0+

[swissspidy]

Task Description

As per #7551, the new minimum required PHP version will be 7.0.

Needs updating in two places:

https://github.com/google/web-stories-wp/blob/c4b2f02b98713b17962f283a5f2205f93d56fbce/composer.json#L8

https://github.com/google/web-stories-wp/blob/c4b2f02b98713b17962f283a5f2205f93d56fbce/composer.json#L43

Followed by a composer update of course.

Perhaps this even allows us to update some packages which have newer versions that aren't compatible with 5.6.

composer outdated should tell us.