fix: update subnet group to fix name vs. name_prefix

[bryantbiggs]

Description

• remove use of var.identifier from sub-module and instead rely on var.name
• allow for either name or name_prefix in the subnet sub-module; currently defaults name_prefix

Motivation and Context

• working towards consistent use of name versus name_prefix for the sub-modules as well as in the root module. This is a continuation of fix: update option group to fix name vs. name_prefix and conditional creation for postgresql #302 and fix: update parameter group to fix name vs. name_prefix #304 which should now conclude the standardization of name and name_prefix for the sub-modules

Closes #42
Closes #48
Closes #121
Closes #209
Closes #223
Closes #250
Closes #280
Closes #290

Breaking Changes

No - other than description change below, there should be no changes for users

For users who are looking to upgrade, they should perform the following change if they wish to avoid any changes:

	...
	# Set the following description
	db_subnet_group_description = "Database subnet group for ${var.identifier}"
	...

How Has This Been Tested?

• I have tested and validated these changes using one or more of the provided examples/* projects
• Tested and validated using the update examples/complete-mysql and `examples/complete-postgresql projects

[bryantbiggs]

@antonbabenko let me know if you have any questions/concerns on this one

[antonbabenko]

Looks good (I just missed this one among hundreds of others github sends me) :)

v2.28.0 has been just released.

[github-actions]

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