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 https://github.com/Orange-OpenSource/Orange-Boosted-Bootstrap/pull/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 https://github.com/twbs/bootstrap/pull/29315 from time to time.

It is maybe related to:

• https://github.com/pa11y/pa11y-ci/issues/128
• https://github.com/pa11y/pa11y/issues/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 https://github.com/Orange-OpenSource/Orange-Boosted-Bootstrap/pull/924.

[louismaximepiton]

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