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/routing/default.go

@@ -1,15 +1,12 @@
 package routing
 
 import (
-	"errors"
 	"fmt"
 	"net/netip"
 
 	"github.com/qdm12/gluetun/internal/netlink"
 )
 
-var ErrRouteDefaultNotFound = errors.New("default route not found")
-
 type DefaultRoute struct {
 	NetInterface string
 	Gateway      netip.Addr
@@ -60,7 +57,7 @@ func (r *Routing) DefaultRoutes() (defaultRoutes []DefaultRoute, err error) {
 	}
 
 	if len(defaultRoutes) == 0 {
-		return nil, fmt.Errorf("%w: in %d route(s)", ErrRouteDefaultNotFound, len(routes))
+		return nil, fmt.Errorf("default route not found: in %d route(s)", len(routes))
 	}
 
 	return defaultRoutes, nil