Update scalafix to 0.10.4

[guilgaly]

When using this plugin on a build using the latest scalafix version, we get a warning that rules were compiled with an obsolete version. In practice the update doesn't seem to make a difference to the features used here, but updating avoids the warning.

I also included:

• updated scalafmt (required a couple config changes and fixed a couple formatting issues)
• an attempt to fix the mergify config as strict mode has been removed after being deprecated (https://blog.mergify.com/strict-mode-deprecation/)

[slandelle]

Hi @NeQuissimus ,

Do you think this has a chance to be merged and released?

[slandelle]

@NeQuissimus gentle ping.