[CodeHealth] Migrate onc_validator to v2 value-based API

Replace deprecated v1 base::Value usage with v2 value-based API
in onc_validator code.

This change mainly replaces `base::Value` usage where type are
deterministic to narrower type such as `base::Value::List` and
`base::Value::Dict`.

Bug: 1303949
Change-Id: I9b78dc3dffd83b9754d5f3c98d811103517a229b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4179389
Commit-Queue: Charlie Hu <chenleihu@google.com>
Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1096580}