No longer compatible with macos 10.15 - update system requirements on your website

- [x] I am on the [latest](https://github.com/ActivityWatch/activitywatch/releases/latest) ActivityWatch version.
- [x] I have searched the issues of this repo and believe that this is not a duplicate.

- **OS name and version**: macos 10.15.7
- **ActivityWatch version**: v0.13.2

## Describe the bug
The front page of your website says ActivityWatch is compatible with macos 10.15+. As of v0.13.2, this is no longer true: `Sorry, "ActivityWatch" cannot be run on this version of macOS. Qt requires macOS 11.0.0 or later, you have macOS 10.15.7.`

## To Reproduce
1. Use macos Catalina
2. Attempt to launch ActivityWatch

## Expected behavior
ActivityWatch runs.