fix: Move guzzle requirement to core

[imorland]

I just had an issue which caused 500 errors when sending mail, only using the mailgun driver. Turns out, we've always had a dependency on GuzzleHttp\Client, but not required it in composer.json

framework/framework/core/src/Mail/MailgunDriver.php

Line 13 in 29179e2

use GuzzleHttp\Client;

Changes proposed in this pull request:
Remove GuzzleHttp/Guzzle from flarum/askimet, and onto core itself.

Necessity

• Has the problem that is being solved here been clearly explained?
• If applicable, have various options for solving this problem been considered?
• For core PRs, does this need to be in core, or could it be in an extension?
• Are we willing to maintain this for years / potentially forever?

Confirmed

• Frontend changes: tested on a local Flarum installation.
• Backend changes: tests are green (run composer test).
• Core developer confirmed locally this works as intended.
• Tests have been added, or are not appropriate here.

Required changes:

• Related documentation PR: (Remove if irrelevant)
• Related core extension PRs: (Remove if irrelevant)