Change pa11y to run on pull requests

[YvonneKallberg]

Update accessibility testing (via pa11y_test.yaml) so that it runs automatically on pull requests instead of (?) running every Friday. '?' since I'm unsure but think that if we run on every pull request we don't have to run weekly. By running on PR:s we get notified immediately if we have introduced an accessibility 'error', which makes it easier to fix fast and not risking that the 'error' is deployed on website.

[YvonneKallberg]

Had a chat with Senthil regarding when running pa11y (for dsw appendix) and he said that one argument to run on weekly basis rather than on pull requests is if there is low frequencies on PRs (no development done) then the requirements on accessibility might change without one becoming aware until long time afterwards That is no risk for us, since the guidelines are actively developed, so I say we go ahead and change to run on PRs (only).