Add PHP 8.0 support

[okdewit]

The official Elasticsearch PHP client now supports PHP 8.0 since v7.11.

I thought it might be a good idea to update laravel-elasticsearch as well.

I have:

• Updated composer.json.
• Added PHP 8.0 to the Github Actions test matrix.
• Added Mockery 1.4.3 as a minimum to require-dev, because 1.3.* had some semver-unfriendly PHP 8 incompatibilities.

The Github Actions test workflow completes succesfully on the fork for all PHP/Laravel combinations.

[cviebrock]

Thanks @okdewit ... I pushed a bunch of changes to add PHP8 support before I noticed your PR that does the same thing!

[okdewit]

@cviebrock Thanks for your continued efforts on supporting this package!