Analyse vimeo/psalm 5.20 and up (now sticky to 5.19.x)

[stronk7]

Because vimeo/psalm 5.20 comes with some new (crazy?) RiskyTruthyFalsyComparison checks that are impacting us a lot, we have changed the dev requirements to use vimeo/psalm 5.19 instead.

This is the PR that introduced the new checks: vimeo/psalm#10502, for examples and friends.

This issue is about to analyse the impact of that new check and decide about to disable it or go fixing all the cases it's reporting (some of them look like "too-much" pedantic, IMO).

For example, see this run using the newer 5.20 version: https://github.com/stronk7/moodle-plugin-ci/actions/runs/8191901373

Ciao :-)