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/openvpn/pkcs8/upgrade.go

@@ -2,14 +2,11 @@ package pkcs8
 
 import (
 	"encoding/base64"
-	"errors"
 	"fmt"
 
 	pkcs8lib "github.com/youmark/pkcs8"
 )
 
-var ErrUnsupportedKeyType = errors.New("unsupported key type")
-
 // UpgradeEncryptedKey eventually upgrades an encrypted key to a newer encryption
 // if its encryption is too weak for Openvpn/Openssl.
 // If the key is encrypted using DES-CBC, it is decrypted and re-encrypted using AES-256-CBC.