provider/aws: Force lowercasing for DB Option group name or name_prefix

[catsby]

Continue #14192 and require users to specify name and name_prefix as lower case strings, to match AWS style. Currently the DB Option Group tests are failing because the name attribute is also used as the id, however the id is not receiving the lowercasing. As a result, the id locally used for lookups does not match the casing used to create the resource, so lookup fails.

Here we modify the validation to error at the plan stage if non-lowercase is used. As it stands, I believe any upper or mixed case name or name_prefix is hitting #11040

The strings.ToLower may be superfluous with the validation, but it felt like good measure 👍

TF_ACC=1 go test ./builtin/providers/aws -v -run=TestAccAWSDBOptionGroup -timeout 120m
=== RUN   TestAccAWSDBOptionGroup_importBasic
--- PASS: TestAccAWSDBOptionGroup_importBasic (14.92s)
=== RUN   TestAccAWSDBOptionGroup_basic
--- PASS: TestAccAWSDBOptionGroup_basic (11.15s)
=== RUN   TestAccAWSDBOptionGroup_namePrefix
--- PASS: TestAccAWSDBOptionGroup_namePrefix (11.58s)
=== RUN   TestAccAWSDBOptionGroup_generatedName
k--- PASS: TestAccAWSDBOptionGroup_generatedName (11.31s)
=== RUN   TestAccAWSDBOptionGroup_defaultDescription
--- PASS: TestAccAWSDBOptionGroup_defaultDescription (11.75s)
=== RUN   TestAccAWSDBOptionGroup_basicDestroyWithInstance
--- PASS: TestAccAWSDBOptionGroup_basicDestroyWithInstance (556.28s)
=== RUN   TestAccAWSDBOptionGroup_OptionSettings
--- PASS: TestAccAWSDBOptionGroup_OptionSettings (20.26s)
=== RUN   TestAccAWSDBOptionGroup_sqlServerOptionsUpdate
--- PASS: TestAccAWSDBOptionGroup_sqlServerOptionsUpdate (20.02s)
=== RUN   TestAccAWSDBOptionGroup_multipleOptions
--- PASS: TestAccAWSDBOptionGroup_multipleOptions (11.75s)
PASS
ok      github.com/hashicorp/terraform/builtin/providers/aws    669.051s

[grubernaut]

LGTM

[ghost]

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