Upgrade Psalm to 6.13

[stof]

Now that we don't need to support PHP 7 anymore, we can upgrade Psalm to version 6.

I disabled some rules in config, especially given Psalm is now stricter about the types of SimpleXMLElement (which returns null for properties when accessing them on an element that does not exist in its parent)

[jderusse]

approved, even if I don't like writing code to please a tool that reports false positive.
(The method will never return null, with the provided arguments)

[stof]

@jderusse knowing with 100% certainty that a regex won't ever produce null is hard due to the JIT backtracking limit. My analysis is based on knowing the exact regex being used and knowing which kind of input we give it (for the S3Client case, where the regex itself could backtrack but our own usage always passes a matching input generated by the parent method), which is not something tools can determine.

Ignoring errors instead of doing an assert (which is a no-op in prod) would be a lot harder as errors are not reported at this location but on next lines having to deal with string|null