fix: remove defaults on backtrackwindow

[quixoticmonk]

Issue #, if available:

Original issue : https://github.com/hashicorp/terraform-provider-awscc/issues/1805

Description of changes:

• The change here is to ensure that the default value of BacktrackWindow is removed so that the schema referenced by entities like AWSCC provider from Terraform do not send this for every DB engine. Opening this based on the internal ticket opened with the team and their suggestions.

• The default behaviour - if BacktrackWindow is missing/null - is for backtrack to be disabled.

• Setting BacktrackWindow to 0 also disables backtrack.

• BacktrackWindow can only be specified if the engine version supports the backtrack feature, otherwise you get the "Backtrack is not enabled" error.

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[zrfr]

I've modified the PR to remove only the default (and not the minimum).