chore: replace interface{} with any

[benhoyt]

any is (as of Go 1.18) a builtin alias for interface{}, equivalent in every respect. It was introduced in Go 1.18+, and our go.mod says we're on 1.22. A similar thing was done to the Go project and stdlib a while back, see for example https://pkg.go.dev/encoding/json#Marshal

Commands used to generate this PR (based on the same thing done to the Go project):

$ sed -i 's,interface{},any,g' **/*.go
$ sed -i 's,interface{},any,g' **/**/*.go
$ sed -i 's,interface{},any,g' **/**/**/*.go
# I'm not actually sure why the last two were needed here and not for Go,
# but the first one only did the client package

go fmt ./...

This also adds a GitHub Action (part of the format checks) that runs grep to ensure we don't have any remaining interface{}, for example from in-flight PRs. See failing example run.

[benhoyt]

@flotter @pedronis What do you think about doing this to snapd and other codebases derived from Pebble, seeing we like to keep in sync?

[flotter]

@benhoyt I do not have a strong opinion other than the change does not bother me (it makes sense to follow what others are doing for me), and if you do it, we will follow.