Set back `concurrency` to `4` in `.pa11yci.json`

[julien-deramond]

pa11y CI was not stable so we set concurrency to 1 in .pa11yci.json via #761.

With this value, the execution is really slow but works "better" (1/4 of executions are green...).

• After each bump of a new version of pa11y-ci dev dependency, try to set back the concurrency to 4.
• Check Add pa11y-ci for automated accessibility tests twbs/bootstrap#29315 from time to time.

It is maybe related to:

• Page crash errors when running against a Jekyll site pa11y/pa11y-ci#128
• Puppeteer Error: Navigation failed because browser has disconnected! pa11y/pa11y#570

[julien-deramond]

We could try https://github.com/nektos/act (thanks @ffoodd for the link)

[julien-deramond]

On hold. We wait to see if it is better after #924.

[louismaximepiton]

For what I'se seen, this doesn't change anything with pa11y-ci@3.0.1. Is this issue still a thing ?