Update to MITK v2021

[alonsoJASL]

We need to update to MITK 2021. Branch feature/mitk-upgrade already is ready to be deployed. I suggest we take some time to:

• test feature/mitk-upgrade builds (write documentation if there's any changes to the build instructions)
• test the app works in all platforms
• merge development into feature/mitk-upgrade to get the latest changes
• merge to master into v3 or CemrgApp

Additionally, this will help with other errors, such as #29

[JostMigenda]

We were discussing on Wednesday at our regular meeting whether to proceed with MITK 2021.x or skip it and go directly to a later release.

Just a note: While looking up the Qt upgrade options for #77, I noticed that the Qt version listed as a pre-requisite in MITK docs is Qt 5.12 up to and including MITK 2023.12; so it looks like that won’t introduce any constraints on our MITK upgrade path.