Explain Ubuntu deploy version

Dockerfile

@@ -45,6 +45,8 @@ COPY test/unittest_data /usr/src/unittest_data
 # Linux build & deployment #
 #                          #
 ############################
+# linuxdeployqt requires the use of the oldest still-supported Ubuntu LTS release,
+# which is 20.04 at the time of writing (2024-03).
 FROM ubuntu:20.04 AS linux-deploy
 
 RUN apt-get update && apt-get install -y \