Adjust the Travis tests to current php and moodle versions

[stronk7]

Not much to say, let's verify that:

• The dev branch (main) works with its highest supported PHP version (8.3).
• The latest stable branch (403_STABLE) works with its highest supported PHP version (8.2).
• The oldest stable branch (39_STABLE) works with the lowest supported PHP version (7.4).

Link to travis PR run: https://app.travis-ci.com/github/moodlehq/moodle-plugin-ci/builds/269415755

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.04%. Comparing base (995a05d) to head (529019f).

Additional details and impacted files

@@            Coverage Diff            @@
##               main     #280   +/-   ##
=========================================
  Coverage     88.04%   88.04%           
  Complexity      731      731           
=========================================
  Files            75       75           
  Lines          2234     2234           
=========================================
  Hits           1967     1967           
  Misses          267      267           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[stronk7]

ta, @kabalin !