Drop node v12 support

[devinivy]

Dropping node v12 support. There's a good discussion in #4279 that outlines other steps we may want to take as we end support for node v12. Here's my approach here:

• I adopted nullish coalescing and optional chaining.
• Recognizing that it might break code that relies on the old behavior, I did adopt ?? in the place of || when evaluating non-boolean expressions. I am open to other thoughts, but I am of the mind that it usually gives a better, more precise behavior than ||. In some cases I kept || when I thought it made sense, e.g to fall through empty header values of ''.
• I did not address the usage of private methods since hapi treats private methods as private to hapi and not just the class in question. One great example is request._log() which is called all over the place. I feel that we should adopt private methods in a given class only if we're willing to remove usage of _-prefixed methods from that class that may be used by other classes.
• I addressed usage of any deprecated APIs I could find: res.writeableEnd rather than res.finished, req.socket rather than req.connection.
• I did not implement anything with AbortSignal since I didn't want to make any substantive API changes, though I am in support of it.

This PR will not pass CI until hapijs/shot#144 and hapijs/lab#1036 are published. I intend to publish beta versions of these (and other) modules in the near future.

Resolves #4279

[devinivy]

I was able to pull in those beta versions of lab, code, and shot. We officially have a passing build of hapi on node v18 :)

I ran into a little snag along the way that is now fixed. Since we've started using req.socket in lieu of the deprecated req.connection, adding req.socket to shot actually put us down some new codepaths in hapi. Previously these code paths were exercised by shot injection requests, which were always missing req.socket. As a result I needed to make shot's req.socket more true to the Net.Socket interface (hapijs/shot#145), and I needed to add a new test to hapi for when req.socket is missing.

[geek]

Looks good! I tested on a few of the main plugins as well, no issues.