ci: give "Merge Approved PR" a merge-method input

[HowardBraham]

Used in MetaMask/metamask-extension#39116

Note

Introduces a configurable merge strategy to the merge-approved-pr.yml workflow.

• Adds optional merge-method input (default merge; supports merge or squash) with validation in Validate Inputs
• Updates merge step to pass merge_method: '${{ inputs.merge-method }}' to github.rest.pulls.merge
• Adjusts logging/messages to include selected merge method

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