Merge branch 'release-2.9' into 'master'

Release 2.9

It is that time of the year again.

See [CHANGELOG.md](https://gitlab.federez.net/re2o/re2o/-/blob/release-2.9/CHANGELOG.md).

Many thanks to the people who made Re2o2.9 possible, thanks to their 575 commits in almost one year of development!

* Jean-Romain Garnier
* Gabriel Detraz
* Hugo Levy-Falk
* Arthur Grisel-Davy
* Corentin Canebier
* Yoann Piétri
* Jean-Marie Mineau
* Leila Bekaddour
* Maxime Bombar

See merge request re2o/re2o!592
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