Fix duplicate PrAT and GrAT expiry emails

What does this MR do and why?

Fix duplicate PrAT and GrAT expiry emails

Users reported receiving duplicate emails for expiring PrAT and GrAT tokens after rolling out new code for the 30d + 60d notification emails. This MR should prevent duplicates by using the cache as a lightweight lock.

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Related to #506438 (closed)