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/provider/windscribe/updater/servers.go

@@ -2,7 +2,6 @@ package updater
 
 import (
 	"context"
-	"errors"
 	"fmt"
 	"net/netip"
 	"sort"
@@ -12,8 +11,6 @@ import (
 	"github.com/qdm12/gluetun/internal/provider/common"
 )
 
-var ErrNoWireguardKey = errors.New("no wireguard public key found")
-
 func (u *Updater) FetchServers(ctx context.Context, minServers int) (
 	servers []models.Server, err error,
 ) {
@@ -51,7 +48,7 @@ func (u *Updater) FetchServers(ctx context.Context, minServers int) (
 				if !node.IP3.IsValid() { // Wireguard + Stealth
 					continue
 				} else if wgPubKey == "" {
-					return nil, fmt.Errorf("%w: for node %s", ErrNoWireguardKey, node.Hostname)
+					return nil, fmt.Errorf("no wireguard public key found: for node %s", node.Hostname)
 				}
 
 				server.VPN = vpn.Wireguard