fix: removed skiff (closes #210) (#211)

Hi there - you can remove Skiff from your list since they are now defunct and closing down in 6 months

https://twitter.com/skiffprivacy/status/1756093174806974831
https://skiff.com/data-migration

README.md

@@ -281,7 +281,6 @@ The below email providers are private, end-to-end encrypted (E2EE) and reasonabl
 **[Tutanota](https://tutanota.com/)** | Free and open source email service based in Germany. It has a basic intuitive UI, secure native mobile apps, anonymous signup, and a .onion site. Tutonota has a full-featured free plan or a premium subscription for businesses allowing for custom domains ($12/ month).<br>Tutanota [does not use OpenPGP](https://tutanota.com/blog/posts/differences-email-encryption/) like most encrypted mail providers, instead they use a standardized, hybrid method consisting of a symmetrical and an asymmetrical algorithm (with 128 bit AES, and 2048 bit RSA). This causes compatibility issues when communicating with contacts using PGP. But it does allow them to encrypt much more of the header data (body, attachments, subject lines, and sender names etc) which PGP mail providers cannot do
 **[Mailfence](https://mailfence.com?src=digitald)** | Mailfence supports OpenPGP so that you can manually exchange encryption keys independently from the Mailfence servers, putting you in full control. Mailfence has a simple UI, similar to that of Outlook, and it comes with bundled with calendar, address book, and files. All mail settings are highly customizable, yet still clear and easy to use. Sign up is not anonymous, since your name, and prior email address is required. There is a fully-featured free plan, or you can pay for premium, and use a custom domain ($2.50/ month, or $7.50/ month for 5 domains), where Bitcoin, LiteCoin or credit card is accepted
 **[MailBox.org](https://mailbox.org/)** | A Berlin-based, eco-friendly secure mail provider. There is no free plan, the standard service costs €12/year. You can use your own domain, with the option of a [catch-all alias](https://kb.mailbox.org/display/MBOKBEN/Using+catch-all+alias+with+own+domain). They provide good account security and email encryption, with OpenPGP, as well as encrypted storage. There is no dedicated app, but it works well with any standard mail client with SSL. There's also currently no anonymous payment option
-**[Skiff](https://skiff.com/)** | End-to-end encrypted, open-source, and privacy-first email that also integrates Web3 features such as crypto wallets and decentralized storage. Skiff has a simple and intuitive UI, supports [mobile apps](https://skiff.com/download) on iOS and Android, and requires no personally identifiable information to sign up or create an account. Skiff offers a Pro plan with additional storage space, aliases, custom domains, and more for $8 per month that can be paid using a credit card or with a crypto wallet.
 
 See [OpenTechFund - Secure Email](https://github.com/OpenTechFund/secure-email) for more details.