Bump igniter dependency to 0.5

[JonasGruenwald]

Hello! 👋

I was looking to install the newly released latest version of oban (2.19) into my project, and hit the following dependency resolution conflict:

Because mneme >= 0.10.0 depends on igniter ~> 0.3.76 or ~> 0.4.0 and oban >= 2.19.0 depends on igniter ~> 0.5, mneme >= 0.10.0 is incompatible with oban >= 2.19.0.
And because your app depends on mneme ~> 0.10.1, oban >= 2.19.0 is forbidden.
So, because your app depends on oban ~> 2.19, version solving failed.

Would be great if the dependency constraint could be relaxed, so that mneme can be used alongside packages that require igniter 0.5

[JonasGruenwald]

I couldn't see anything actually breaking in the igniter changelog from 0.4 to 0.5 https://hexdocs.pm/igniter/0.5.8/changelog.html, so I opened a PR to include the new version.