[cleanup] Math/rand.Seed deprecated about gopass HOT 5 OPEN

@dominikschulz
Hi, I'd like to take on this issue if that's okay.

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@dominikschulz
Thank you. I've created a PR. Please check it out:
#2675

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Usually "Deprecated" just means it's finally stable ;)

But I'll check what the recommendation is these days.

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• pkg/pwgen/rand.go This one can Go away once we don't need to support Go 1.20 anymore.
• pkg/fsutil/fsutil.go Same here.

For the test files we actually rely on detemernistic output, so we will need some more changes. We need to introduce a global variable with a local generator and seed it like this New(NewSource(seed)). See https://pkg.go.dev/math/rand#Seed

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@orangekame3 Feel free to do so

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