Bump selenium image to version 4

[kabalin]

This aligns default Selenium version with moodle-docker.

Also patchset adds MOODLE_500_STABLE to GHA/Travis tests to cover latest release.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.31%. Comparing base (d3e0af6) to head (229b485).
Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##               main     #354   +/-   ##
=========================================
  Coverage     88.31%   88.31%           
+ Complexity      762      761    -1     
=========================================
  Files            77       77           
  Lines          2344     2344           
=========================================
  Hits           2070     2070           
  Misses          274      274           

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[andrewnicols]

This currently fails against Travis @kabalin.

I'm increasingly of the opinion that we should drop support for Travis. Do we have any evidence that people actually still use it?

[kabalin]

This currently fails against Travis @kabalin.

I'll merge with unmet requirements (looks like zlib extension is missing in Travis). Thanks for review!

I'm increasingly of the opinion that we should drop support for Travis. Do we have any evidence that people actually still use it?

I agree. No idea if anyone uses it, I guess ticket #351 could make it evident.