Add support for expiring advisory ignores

[abeljim8am]

Allow advisory ignores in .bundler-audit.yml to expire automatically, so temporary vulnerability exceptions do not remain in place indefinitely.

Timed ignores use an advisory ID and an inclusive ISO 8601 until date:

ignore:
  - id: CVE-YYYY-XXXX
    until: 2026-08-31

Existing string entries continue to be ignored indefinitely. Once a timed ignore expires, the advisory is reported normally. Invalid dates, missing fields, and unknown keys are rejected when loading the configuration.

The configuration and scanner specs cover active and expired ignores, backward compatibility, and validation failures.