Add minimum PHP requirement to info.yml, composer.json

Problem/Motivation

The code uses PHP functions that are available in 7.1+, such as return type hints. Drupal Core still supports PHP 7.0.8, so this module is not compatible with the same PHP versions as Drupal core.

Proposed resolution

Add minimum PHP requirement to info.yml, composer.json

Remaining tasks

1. Write a patch
2. Review
3. Commit

User interface changes

None.

API changes

None.

Data model changes

None.