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/models/servers.go

@@ -3,7 +3,6 @@ package models
 import (
 	"bytes"
 	"encoding/json"
-	"errors"
 	"fmt"
 	"math"
 	"reflect"
@@ -158,8 +157,6 @@ type Servers struct {
 	Servers   []Server `json:"servers,omitempty"`
 }
 
-var ErrServersFormatNotSupported = errors.New("servers format not supported")
-
 func (s *Servers) Format(vpnProvider, format string) (formatted string, err error) {
 	switch format {
 	case "markdown":
@@ -167,7 +164,7 @@ func (s *Servers) Format(vpnProvider, format string) (formatted string, err erro
 	case "json":
 		return s.toJSON()
 	default:
-		return "", fmt.Errorf("%w: %s", ErrServersFormatNotSupported, format)
+		return "", fmt.Errorf("servers format not supported: %s", format)
 	}
 }