Add customize diff for deprecated attribute on gc_policy

[modular-magician]

Adding customize diff when google_bigtable_gc_policy.max_age.days is equivalent to google_bigtable_gc_policy.max_age.duration.

resolves #8416

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• Ensured that all new fields I added that can be set by a user appear in at least one example (for generated resources) or third_party test (for handwritten resources or update tests).
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• Read the Release Notes Guide before writing my release note below.

Release Note Template for Downstream PRs (will be copied)

bigtable: fixed bug where gc_policy would attempt to recreate the resource when switching from deprecated attribute but maintaining the same value underlying value

Derived from GoogleCloudPlatform/magic-modules#4556

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