Remove the dependency to the `retain_mut` crate

[Kerollmops]

We are depending on the retain_mut crate which has been deprecated on May 20 2022 as the standard library integrated this function since 1.61. However, we don't want to remove this dependency right now as it would force us to bump the MSRV to 1.61, we were in 1.56.1.

I would like to bump the MSRV to 1.61 but not right now, maybe in 6 months.

/remind me that must remove the retain_mut dependency in 6 months

[reminders[bot]]

@Kerollmops set a reminder for Dec 28th 2022