docs: map FSRS library versions to displayString

[david-allison]

https://forums.ankiweb.net/t/anki-26-05-beta-1/69707/9?u=david FSRS "5.2.0" is still FSRS 6

ankitects/anki#4956 (comment) FSRS "6.3.0" is still FSRS 6

Checklist

• You have a descriptive commit message with a short title (first line, max 50 chars).
• You have commented your code, particularly in hard-to-understand areas
• You have performed a self-review of your own code
• UI changes: include screenshots of all affected screens (in particular showing any new or changed strings)
• UI Changes: You have tested your change using the Google Accessibility Scanner

[david-allison]

Pending this PR (up to 6.6.1)

• chore: bump fsrs version ankitects/anki#4956