Declare PHP requirement >= 7.4

Problem/Motivation

Tests are failing at due to use of typed properties.

At https://www.drupal.org/node/215311/qa → we can see tests for 2.x branch on PHP7.2 are failing due to typed properties such as this. Typed properties are only allowed since PHP 7.4, and the linked example was introduced into this module in #3214737: Display Missing Mailchimp API Key message → .

Simple fix is to declare the requirement for PHP7.4, and update the automated testing configuration to use that version.

Steps to reproduce

Either:

- Visit https://www.drupal.org/node/215311/qa →
- Click the red test result for 2.x branch with PHP7.2

Or, run tests locally using a PHP version below 7.4.

Proposed resolution

Declare requirement for PHP7.4.

Remaining tasks

- [ ] Update test configuration (maintainer action required)
- [x] Update info.yml
- [x] Update composer.json
- [ ] Re-run tests
- [ ] Review, etc

User interface changes

API changes

Data model changes