Review ESR 115 expiry date for legacy OS message

In #43125 (closed) I set back the expiry date to be 2025-04-01. After this date the user will see:

Tor Browser no longer supports this version of Windows. Please upgrade to Windows 10 or later to continue receiving important security updates.

and similar for macOS.

We should review this date in the future in case mozilla and Tor Browser extends the support time.

I set the due date for this issue to be roughly 8 weeks before the expiry date, a month before the last version ESR 115.21 (2025 March 4th).

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