[Part 2] Modernise code

[ryanmab]

Description

Continuation of: #91

This does:

1. Bumps to PHPUnit 10 (preferable for some rector rules - and okay to move to after the changes in Part 1)
2. Applies Rector rules for PHP up to 8.2 (what is now listed in the composer file)
3. Applies Rector rules for PHPUnit up to 10
4. Applies some standard Rector rules for code hygiene (mostly removing docblocks which aren't needed and adding types which are supported)
5. Adds Rector to CI pipeline (to run code hygiene rules)

Testing Notes

Will hook this up to the platform locally before merging and test both Part 1 and 2 together. Once thats done, I'll get merging!

[ryanmab]

Thought I was passed the point of finding new PHP syntax I didn't know, but apparently not. Have double checked, these are definitely equivalent.

[gabriel-breda]

interesting indeed! Apparently it's a first class callable:

same semantics as Closure::fromCallable()

[gabriel-breda]

All good, not much to add.
I guess there is a bit of risk with the added type, in case we weren't following exactly what we're supposed to have. But it's good to have them and we can hope the tests would catch those cases before going to prod 🤞

[gabriel-breda]

definitely controversial! 😉

[ryanmab]

This is the most important part of this PR! 😜