Updating tools/mafft from version 7.508 to 7.520

[planemo-autoupdate]

Hello! This is an automated update of the following tool: tools/mafft. I created this PR because I think the tool's main dependency is out of date, i.e. there is a newer version available through conda.

I have updated tools/mafft from version 7.508 to 7.520.

For any comments, queries or criticism about the bot, not related to the tool being updated in this PR, please create an issue here.