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/constructor_test.go

@@ -1,6 +1,7 @@
 package wireguard
 
 import (
+	"errors"
 	"net/netip"
 	"testing"
 
@@ -25,7 +26,7 @@ func Test_New(t *testing.T) {
 			settings: Settings{
 				PrivateKey: "",
 			},
-			err: ErrPrivateKeyMissing,
+			err: errors.New("private key is missing"),
 		},
 		"minimal valid settings": {
 			settings: Settings{