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/wireguard/config.go

@@ -1,6 +1,7 @@
 package wireguard
 
 import (
+	"errors"
 	"fmt"
 	"net"
 	"net/netip"
@@ -27,19 +28,19 @@ func ConfigureDevice(client *wgctrl.Client, settings Settings) (err error) {
 func makeDeviceConfig(settings Settings) (config wgtypes.Config, err error) {
 	privateKey, err := wgtypes.ParseKey(settings.PrivateKey)
 	if err != nil {
-		return config, ErrPrivateKeyInvalid
+		return config, errors.New("cannot parse private key")
 	}
 
 	publicKey, err := wgtypes.ParseKey(settings.PublicKey)
 	if err != nil {
-		return config, fmt.Errorf("%w: %s", ErrPublicKeyInvalid, settings.PublicKey)
+		return config, fmt.Errorf("cannot parse public key: %s", settings.PublicKey)
 	}
 
 	var preSharedKey *wgtypes.Key
 	if settings.PreSharedKey != "" {
 		preSharedKeyValue, err := wgtypes.ParseKey(settings.PreSharedKey)
 		if err != nil {
-			return config, ErrPreSharedKeyInvalid
+			return config, errors.New("cannot parse pre-shared key")
 		}
 		preSharedKey = &preSharedKeyValue
 	}