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