Enhancement/#11449 - Set isGTGDefault based on existing settings

[hussain-t]

Summary

Addresses issue:

• Implement Google Tag Gateway automatic enablement heads-up banner #11449
• Re-surface GTG auto-enablement preparation #11502
• Implement backward compatibility with isGTGDefault setting for existing GTG enabled sites #11504

Relevant technical choices

Problem

• Existing sites with GTG enabled were receiving isGTGDefault = true, making them eligible for opt-out notices, even though they had already chosen to enable GTG.

Solution

• This follow-up PR adds logic to set isGTGDefault = false for existing sites where GTG was previously enabled.
• This ensures opt-out notices only show to users who haven't explicitly interacted with GTG settings.

PR Author Checklist

• My code is tested and passes existing unit tests.
• My code has an appropriate set of unit tests which all pass.
• My code is backward-compatible with WordPress 5.2 and PHP 7.4.
• My code follows the WordPress coding standards.
• My code has proper inline documentation.
• I have added a QA Brief on the issue linked above.
• I have signed the Contributor License Agreement (see https://cla.developers.google.com/).

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Code Reviewer Checklist

• Run the code.
• Ensure the acceptance criteria are satisfied.
• Reassess the implementation with the IB.
• Ensure no unrelated changes are included.
• Ensure CI checks pass.
• Check Storybook where applicable.
• Ensure there is a QA Brief.
• Ensure there are no unexpected significant changes to file sizes.

Merge Reviewer Checklist

• Ensure the PR has the correct target branch.
• Double-check that the PR is okay to be merged.
• Ensure the corresponding issue has a ZenHub release assigned.
• Add a changelog message to the issue.

[github-actions]

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