[RDS]: fix keep_days update for opentelekomcloud_rds_instance_v3

[artem-lifshits]

Summary of the Pull Request

Fix keep_days update when changing from 1 to 0 value.

PR Checklist

• Refers to: RDSv3: Error "missing input for argument [KeepDays]" when keep_days is 0 #2262
• Tests passed.
• Release notes added.

Acceptance Steps Performed

=== RUN   TestAccRdsInstanceV3Basic
--- PASS: TestAccRdsInstanceV3Basic (762.16s)
=== RUN   TestAccRdsPostgre13V3ParamsBasic
--- PASS: TestAccRdsPostgre13V3ParamsBasic (533.63s)
=== RUN   TestAccRdsInstanceV3ElasticIP
--- PASS: TestAccRdsInstanceV3ElasticIP (761.48s)
=== RUN   TestAccRdsInstanceV3OptionalParams
--- PASS: TestAccRdsInstanceV3OptionalParams (394.23s)
=== RUN   TestAccRdsInstanceV3Backup
--- PASS: TestAccRdsInstanceV3Backup (359.66s)
PASS

Process finished with exit code 0

[otc-zuul]

Build succeeded.
https://zuul.otc-service.com/t/eco/buildset/552ffb88da49400fbc80aba058740139

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