Travis CI: migrate to Ubuntu Focal Fossa 20.04 LTS

[php-coder]

See https://blog.travis-ci.com/2020-08-10-focal-build-environment and https://docs.travis-ci.com/user/reference/focal/
Highlights:

• Java 10 and Java 11 (and common builds tools: gradle, maven, groovy) (see also Make the project at least compilable on newer JDKs #1350 and Migrate to JDK 11 #1158)
• PostgreSQL 12.2
• MySQL 8.0.21 (see also Docker: update MySQL to 5.7.27 #1017 and Update mysql-connector-java to 5.1.49 #1184)
• Python 3.6