remove the custom retention days field as the feature is not GA

[dubey-vivek]

cloudsql: remove `max_custom_on_demand_retention_days` field from `sqladmin` resource

[github-actions]

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[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.

google provider: Diff ( 2 files changed, 2 deletions(-))
google-beta provider: Diff ( 2 files changed, 2 deletions(-))

[modular-magician]

Tests analytics

Total tests: 35
Passed tests: 22
Skipped tests: 12
Affected tests: 1

Click here to see the affected service packages

• backupdr

Action taken

Found 1 affected test(s) by replaying old test recordings. Starting RECORDING based on the most recent commit. Click here to see the affected tests

• TestAccBackupDRBackupPlan_backupDrBackupPlanForCsqlResourceExample

Get to know how VCR tests work

[modular-magician]

🟢 Tests passed during RECORDING mode:
TestAccBackupDRBackupPlan_backupDrBackupPlanForCsqlResourceExample [Debug log]

🟢 No issues found for passed tests after REPLAYING rerun.

---

🟢 All tests passed!

View the build log or the debug log for each test