Drop support for PHP 7.1

[askvortsov1]

**Fixes #1988 **

Changes proposed in this pull request:

• Drop support for PHP 7.1
• Use v1 of wikimedia/less (more up to date)

Reviewers should focus on:

• Wikimedia less is planning to release v3.0.0 shortly (Tag v3.0.0 wikimedia/less.php#26) to clear up their versioning. I think we shouldn't merge till then, and update our composer.json when they do.
• In Require PHP 7.2+ #1988 (comment), Matt suggested upgrading Laravel to 6.x. When I tried doing this, our automated tests would fail when ran in github actions (although not on my local machine). Do we want to look into this update and compatibility with our setup? If so, here, or in a second PR?

[dsevillamartin]

No to Laravel v6 - that will be a separate PR, most likely created by a core member as it requires changes to Flarum's internals.

[franzliedke]

I'd prefer not to delay this by waiting for the LESS library - can you remove the change to that library so that we can merge this now?

[askvortsov1]

I'd prefer not to delay this by waiting for the LESS library - can you remove the change to that library so that we can merge this now?

Done!