Add PHP 7.2 CI tests

[mmachatschek]

This adds PHP 7.2 CI tests.

The composer.json tells us, that the version is supported but we do not actually test this.

PS: If we want to upgrade the minimum dependency to php:^7.3|^8.0 I would gladly do so. PHP 7.2 is not receiving any bugfixes and security fixes anymore and we would encorage the users to upgrade their stack to a supported version.

[shokme]

@mmachatschek we can keep the 7.2 like the sdk does: https://github.com/meilisearch/meilisearch-php/issues/136

[mmachatschek]

Also, to merge your PR, I need you to update the bors.toml file (https://github.com/meilisearch/meilisearch-laravel-scout/blob/master/bors.toml) by adding the 'tests (php 7.2)' in the status field blush

@curquiza done.

I agree with @shokme, let's keep the 7.2 support for the moment. If you disagree with this choice, an issue (meilisearch/meilisearch-php#136) is available here (I will reopen it if we need to debate) slightly_smiling_face We like having the community opinion!

No need for that. I agree as long as everything aligns with the dependency tree (laravel/scout and meilisearch/meilisearch/php) :+1:

[bors[bot]]

Build succeeded:

• linter-check
• tests (php 7.2)
• tests (php 7.3)
• tests (php 7.4)
• tests (php 8.0)