[RDS] Unify usage of tags

[lego963]

Summary of the Pull Request

Unify usage of tags in resource/opentelekomcloud_rds_instance_v3

Closes: #918

PR Checklist

• Refers to: Unify usage of tags in resources #918
• Tests added/passed.
• Documentation updated.
• Schema updated.

Acceptance Steps Performed

=== RUN   TestAccRdsInstanceV3Basic
--- PASS: TestAccRdsInstanceV3Basic (1183.81s)
=== RUN   TestAccRdsInstanceV3ElasticIP
--- PASS: TestAccRdsInstanceV3ElasticIP (705.28s)
=== RUN   TestAccRdsInstanceV3OptionalParams
--- PASS: TestAccRdsInstanceV3OptionalParams (649.11s)
=== RUN   TestAccRdsInstanceV3Backup
--- PASS: TestAccRdsInstanceV3Backup (688.16s)
=== RUN   TestAccRdsInstanceV3TemplateConfig
--- PASS: TestAccRdsInstanceV3TemplateConfig (723.68s)
=== RUN   TestAccRdsInstanceV3InvalidDBVersion
--- PASS: TestAccRdsInstanceV3InvalidDBVersion (3.67s)
PASS

Process finished with the exit code 0

[otc-zuul]

Build succeeded.

• golang-make-test : SUCCESS in 3m 55s
• golang-make-vet : SUCCESS in 8m 06s
• goreleaser-build : SUCCESS in 9m 29s