AWS::ElastiCache::ParameterGroup should support updates

## Scope of request
AWS::ElastiCache::ParameterGroup is already supported as an immutable resource. The resource should be extended to support updates.

## Expected behavior
In Update, it shouldn't fail. Current behavior is:

> Update to resource type AWS::ElastiCache::ParameterGroup is not supported.

## Links to existing API doc
https://docs.aws.amazon.com/AmazonElastiCache/latest/red-ug/ParameterGroups.Modifying.html
https://docs.aws.amazon.com/AmazonElastiCache/latest/mem-ug/ParameterGroups.Modifying.html

## Category
DB

Read the linked ElastiCache Parameter Groups modifying documentation and locate the AWS::ElastiCache::ParameterGroup Update entry point in the implementation. Confirm the current unsupported-update path, then verify that supported parameter changes complete successfully without the reported error.

Written by the indexing model from the issue text.