Move prompts to stderr

[dcousens]

I think it would be better to print prompts like

age-plugin-tpm/cmd/age-plugin-tpm/main.go

Lines 90 to 100 in 5dedd8a

	pin, err = GetPin("Enter pin for key:")
	if err != nil {
	return err
	}
	clearLine(os.Stdin)
	confirm, err := GetPin("Confirm pin:")
	if err != nil {
	return err
	}

to stderr, as is common in POSIX applications.

This became problematic for me when I was trying the following:

age-plugin-tpm --generate --pin > test.tpm

And the prompt didn't show

[dcousens]

Actually, I don't know what the convention is, but stderr seems an easier option than stdout?

[Foxboron]

I don't think stderr is appropriate, but I'll research it a bit.