docs: map past FSRS library versions

[david-allison]

A user was asking about this, so I mapped AnkiDroid 2.20 and 2.21's version for posterity.

AnkiDroid 2.18 did not list the version on the 'About' page

FSRS 0.6.4 = AnkiDroid 2.19 = Anki 24.06.3 = FSRS 4.5
FSRS 1.4.3 = AnkiDroid 2.20 = Anki 24.11   = FSRS 5
FSRS 2.0.3 = AnkiDroid 2.21 = Anki 25.02   = FSRS 5

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

[BrayanDSO]

only a nit