set rdb-snapshot-period to optional

[googlerjk]

Updated field rdb_snapshot_period to optional in the persistance_config redis resource.

Fixes hashicorp/terraform-provider-google#12854

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• Searched through the issue tracker for an open issue that this either resolves or contributes to, commented on it to claim it, and written "fixes {url}" or "part of {url}" in this PR description. If there were no relevant open issues, I opened one and commented that I would like to work on it (not necessary for very small changes).
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Release Note Template for Downstream PRs (will be copied)

redis: Updated`rdb_snapshot_period` to optional in the `persistence_config` resource.

[modular-magician]

Hi there, I'm the Modular magician. I've detected the following information about your changes:

Diff report

Your PR generated some diffs in downstreams - here they are.

Terraform GA: Diff ( 2 files changed, 3 insertions(+), 3 deletions(-))
Terraform Beta: Diff ( 2 files changed, 3 insertions(+), 3 deletions(-))
TF Validator: Diff ( 2 files changed, 3 insertions(+), 3 deletions(-))

[modular-magician]

Tests analytics

Total tests: 2207
Passed tests 1968
Skipped tests: 239
Failed tests: 0

All tests passed in REPLAYING mode
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