You need to upgrade to 1.2 first, then to 2.0, then to 2.1.
The first part is the most problematic one, since 1.2 is broken - it’s many years old, uses now-obsolete packages, and some of these packages fail to install due to changes in the Python package infrastructure.
However, there are some workarounds in this post to install 1.2 in a way that is good enough to migrate the 1.1 DB to 1.2.
Note: If you were not using the “Call for Abstracts” module in 1.1, you could try skipping 1.2 altogether - it will most likely work and save you some pain from setting up 1.2 first.
Once you are ready to migrate, follow these steps: https://docs.getindico.io/en/latest/installation/upgrade_legacy/
Note: If you go through 1.2 first, do it on some other VM you can get rid of afterwards, or a separate virtual machine. Installing 1.2 will install a lot of stuff in your Python virtualenv that is outdated and not needed for Indico 2.x.
Once you have 2.0 running, follow https://docs.getindico.io/en/latest/installation/upgrade/ to get to 2.1.