upgrade phpstan to 0.12 and thecodingmachine/safe

[bendavies]

Summary
Upgrade thecodingmachine/safe and phpstan+deps to the latest releases.

This allows using thecodingmachine/gotenberg-php-client:^6.1 in projects that also want use thecodingmachine/safe:^1.0 and phpstan/phpstan:^0.12+ thecodingmachine/phpstan-safe-rule^1.0.
Without this change, projects are stuck on phpstan/phpstan:^0.11

Test plan (required)

increased phpstan level in accordance with upgrade notes. make lint still passes.

Closing issues

Fixes #

Checklist

• Have you followed the guidelines in our CONTRIBUTING guide?
• Have you lint your code locally prior to submission (make lint)?
• Have you written new tests for your core changes, as applicable?
• Have you successfully ran tests with your changes locally (make tests)?
• I have squashed any insignificant commits
• This change has comments for package types, values, functions, and non-obvious lines of code

[gulien]

Thank you very much @bendavies

[gulien]

Version 6.1.1 is released 🎉