src: migrate off ArrayBuffer::GetContents

V8 deprecates `GetContents()` in favour of `GetBackingStore()`.
Update our code to reflect that.
V8 also deprecates `Externalize()` and `IsExternal()`; we should
be able to remove all usage of this once V8 8.0 is there.

PR-URL: https://github.com/nodejs/node/pull/30339
Refs: https://github.com/v8/v8/commit/bfe3d6bce734e596e312465e207bcfd55a59fe34


Reviewed-By: Gus Caplan <me@gus.host>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Jiawen Geng <technicalcute@gmail.com>
Reviewed-By: David Carlier <devnexen@gmail.com>