A good measurement should always estimate between the smallest markings. In this case, the smallest markings are 1 mL, so the measurement should be recorded as 21.5 mL or maybe 21.7 mL if you think it's a bit higher than halfway. If it were right on the 21 mL line, it should be recorded as 21.0 mL.
However, do not go too far. You have no clue what the hundredths place is here. 21.50 mL would just be making things up.
Visual Guide to Unit Conversion