Bump PHP requirement in plugin

[swissspidy]

Task Description

Need to bump the PHP 5.6 requirement in two places:

https://github.com/google/web-stories-wp/blob/c4b2f02b98713b17962f283a5f2205f93d56fbce/web-stories.php#L17

https://github.com/google/web-stories-wp/blob/c4b2f02b98713b17962f283a5f2205f93d56fbce/web-stories.php#L50

[spacedmonkey]

There is also

Remove from unit tests.

https://github.com/google/web-stories-wp/blob/c4b2f02b98713b17962f283a5f2205f93d56fbce/.github/workflows/tests-unit-php.yml#L48

https://github.com/google/web-stories-wp/blob/c4b2f02b98713b17962f283a5f2205f93d56fbce/.github/workflows/tests-unit-php.yml#L56

Remove it from composer

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

By changing this value in composer.json, we should also build the composer lock file and see what dependencies we can remove / update.

Update coding standards.

On a similar note we can also change this line

https://github.com/google/web-stories-wp/blob/c4b2f02b98713b17962f283a5f2205f93d56fbce/phpcs.xml.dist#L61-L62

We can then review what languages, like Scalar type declarations, Return type declarations and Group use declarations we would want to use. This is an epic in it's self.

[swissspidy]

Yup! I split these all up into different tickets, and turned #7551 into an epic.

Remove from unit tests.

#7599

Remove it from composer

#7594

Update coding standards.

#7595

We can then review what languages, like Scalar type declarations, Return type declarations and Group use declarations we would want to use. This is an epic in it's self.

#7597

[swissspidy]

Note: Do not merge before 1.8.0