add default cooldown for gradle package updates

[Jonopono123]

What

Add default cooldown of 7 days to dependabot.yml for gradle packages.

Why

Following on from recent compromised package updates, we want to enforce a 7 day cooldown on any package updates to ensure that we're not potentially bringing dangerous updates into our deployments.

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Internal change (non-breaking change with no effect on the functionality affecting end users)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

• I have performed a self-review of my code
• My changes generate no new warnings
• New and existing unit tests pass locally with my changes

[github-actions]

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