fix: remove db_option_group for postgres engine, as not in use

[kuzm1ch]

Description

• avoid db_option_group creating in case DB engine is postgres
• minor update example based on it

Motivation and Context

PostgreSQL does not use options and option groups. PostgreSQL uses extensions and modules to provide additional features. For more information, see Supported PostgreSQL features and extensions.

https://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/USER_WorkingWithOptionGroups.html

Breaking Changes

In case "PostgresSQL option" was previosly created, it could some problem while removing it with next apply(cannot be deleted because it is in use).
terrafor state rm can be used as a workaround.

terraform state rm module.dbnew.module.db_option_group.aws_db_option_group.this

How Has This Been Tested?

examples/complete-postgres/main.tf

[gschaffer-cxn]

@antonbabenko can we get a review? This has been an issue and multiple PRs exist already:
#282
#259
#251
#188
#272

[bryantbiggs]

I will take a look today and validate this change @kuzm1ch / @gschaffer-cxn

[github-actions]

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