PHP SDK improvements

[adri]

We're evaluating the PHP SDK and found a few issues we see with this package:

• Depends directly on Guzzle, instead use php-http/discovery. This allows people to use their own compatible client.
• Depends on Monolog, instead use a PSR Log. You can suggest monolog
• Run tests run on a (recent) PHP version matrix, not just 7.4 (this version and 8.0 is unsupported)
• Use phpstan
• 2 days ago lastguest/murmurhash was pulled in that's 6 years old, 2 years last commit. The SDK should require minimum deps.

[bgiori]

Thanks @adri! Appreciate your expertise here.

[ruudk]

With the merge of #13 this can be closed 🎉