Add support for Guzzle 8

[jtojnar]

This is a bit stricter but mostly compatible.

Changes affecting us involve improved type annotations and splitting out ResponseException.

https://github.com/guzzle/guzzle/releases/8.0.0

Q	A
Bug fix?	yes
New feature?	no
BC breaks?	no
Deprecations?	no
Related tickets	fixes #X, partially #Y, mentioned in #Z
Documentation	if this is a new feature, link to pull request in https://github.com/php-http/documentation that adds relevant documentation
License	MIT

What's in this PR?

Explain what the changes in this PR do.

Why?

Which problem does the PR fix? (remove this section if you linked an issue above)

Example Usage

// If you added new features, show examples of how to use them here
// (remove this section if not a new feature)

$foo = new Foo();

// Now we can do
$foo->doSomething();

Checklist

• Updated CHANGELOG.md to describe BC breaks / deprecations | new feature | bugfix
• Documentation pull request created (if not simply a bugfix)

To Do

• If the PR is not complete but you want to discuss the approach, list what remains to be done here