(dev/drupal#79) CRM_Upgrade_Form - Raise MINIMUM_PHP_VERSION from 5.6 to 7.0

(dev/drupal#79) CRM_Upgrade_Form - Raise MINIMUM_PHP_VERSION from 5.6 to 7.0

This version requirement officially went up to PHP 7.0 circa Civi 5.14.
However, at that time, the upgrade metadata was kept at PHP 5.6 to allow somewhat
softer landing for stragglers. That's no longer possible in 5.16+, so this
just updates the requirement.

Before
------

When upgrading via drush to Civi 5.16+ on PHP 5.6, the Civi class-loader fails to initialize.

```
Parse error: syntax error, unexpected ':', expecting '{' in /Users/totten/bknix/build/dmaster/web/sites/all/modules/civicrm/vendor/league/csv/src/functions.php on line 33
```

(Approximate call-path: `civicrm.drush.inc` => `civicrm.settings.php` =>
`CRM_Core_ClassLoader` => `vendor/autoload.php` => `vendor/league/csv/src/functions.php`)

After
-----

It still fails, but at least the metadata is accurate.

There will be a separate patch in another repo to make `drush` use this
metadata and present a better error.