chore: do not use sentinel errors when unneeded

- main reason being it's a burden to always define sentinel errors at global scope, wrap them with `%w` instead of using a string directly
- only use sentinel errors when it has to be checked using `errors.Is`
- replace all usage of these sentinel errors in `fmt.Errorf` with direct strings that were in the sentinel error
- exclude the sentinel error definition requirement from .golangci.yml
- update unit tests to use ContainersError instead of ErrorIs so it stays as a "not a change detector test" without requiring a sentinel error

internal/updater/openvpn/fetch.go

@@ -19,7 +19,7 @@ func FetchFile(ctx context.Context, client *http.Client, url string) (
 	const rejectDomain = false
 	hosts := extractRemoteHosts(b, rejectIP, rejectDomain)
 	if len(hosts) == 0 {
-		return "", fmt.Errorf("%w for url %s", ErrNoRemoteHost, url)
+		return "", fmt.Errorf("remote host not found for url %s", url)
 	}
 
 	return hosts[0], nil