Mention that IVPN accepts prepaid cards

[redoomed1]

trafficstars

Changes proposed in this PR:

• As a continuation of #2563, mention that IVPN accepts prepaid cards like Mullvad does
• As part of the previous change, embed link to IVPN's "Voucher cards FAQ" since they do not advertise this payment method on their "Pricing" page like Mullvad does on theirs (under "Do you offer any type of discount?")
• Make mention of Mullvad's prepaid cards its own sentence for increased readability

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This pull request has been mentioned on Privacy Guides. There might be relevant details there:

https://discuss.privacyguides.net/t/2024-05-22/18443/1