Update securely-using-pull_request_target to remove actions/checkout version restriction

[JackPGreen]

Originally the change applied to actions/checkout v7 only, but it's since been backported to older versions without the docs being updated.

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Source	Review	Production	What Changed
actions/reference/security/securely-using-pull_request_target.md	fpt ghec ghes@ 3.22 3.21 3.20 3.19 3.18 3.17	fpt ghec ghes@ 3.22 3.21 3.20 3.19 3.18 3.17

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[subatoi]

Thank you for spotting that and raising a fix, @JackPGreen—I'll get this merged now

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