rds: Improve version/instance data sources #5099

Job	Run time
go mod download	18s
go mod download	18s
markdown-lint	19s
markdown-lint	19s
go build	1m 53s
go build	1m 53s
Sweeper Functions Not Linked	2m 49s
Sweeper Functions Not Linked	2m 49s
go generate	2m 56s
go generate	2m 56s
go test	9m 34s
go test	9m 9s
importlint	16s
importlint	16s
terraform providers schema	25s
terraform providers schema	25s
tfproviderdocs	19s
tfproviderdocs	19s
	37m 13s

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