docs(samples): Add samples for Cloud Spanner Default Backup Schedules

[Sally-Ye]

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Fixes #1237 🦕

[snippet-bot]

Here is the summary of changes.

You are about to add 2 region tags.

• samples/samples/snippets.py:3225, tag spanner_create_instance_without_default_backup_schedule
• samples/samples/snippets.py:3252, tag spanner_update_instance_default_backup_schedule_type

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[harshachinta]

@Sally-Ye Please fix samples lint.

[Sally-Ye]

@harshachinta Done. Thanks!