fix: Accept the deprecated 3 as an alias for 2

[Hocuri]

fix #3408 by automatically changing a 3 to 2.

3 is a deprecated alias for 2, so all UIs now need to handle both values. I think it wasn't a good idea to deprecate 3 in the first place, but it's not worth it going back now.

I tested this PR like this:

• Without this PR:
  • Set certificate checks to "Accept invalid Certificates" in the settings -> imap_certificate_checks is 2
  • Import a backup from Desktop with "Accept invalid Certificates"
  • Open "Password and Account" in the settings -> the app crashes
• With this PR:
  • Open "Password and Account" in the settings -> the app doesn't crash anymore
  • -> imap_certificate_checks is 3
  • Save the settings
  • -> imap_certificate_checks is 2

[r10s]

did a PR fixing the documentation for the CFFI as well, https://github.com/deltachat/deltachat-core-rust/pull/6176

[github-actions[bot]]

To test the changes in this pull request, install this apk: 📦 app-preview.apk

[r10s]

so that we can sleep better, i created #3412 :)

[r10s]

I think it wasn't a good idea to deprecate 3 in the first place, but it's not worth it going back now.

i agree, lesson learned. we should try harder to keep things stable for UI. we had a similar hickup with DC_QR_BACKUP2 recently.

[Hocuri]

Merging since it fixes a crash