[10.x] Remove workarounds for old Guzzle versions

[GrahamCampbell]

We already require Guzzle ^7.5, which means all the newer (like 3 or 4 years old at this point) class methods are necessarily available.

[nunomaduro]

@GrahamCampbell What do you think about including "conflict", to ensure people are indeed using a Guzzle version that is compatible with Laravel?

    "conflict": {
        "guzzlehttp/guzzle": "<7.6"
    },

[GrahamCampbell]

What do you think about including "conflict"

I typically don't like this, because it stops someone using whatever version of Guzzle they want to use, unrelated to using it through Laravel.