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/custom/openvpnconf.go

@@ -1,7 +1,6 @@
 package custom
 
 import (
-	"errors"
 	"fmt"
 	"strconv"
 	"strings"
@@ -12,8 +11,6 @@ import (
 	"github.com/qdm12/gluetun/internal/provider/utils"
 )
 
-var ErrExtractData = errors.New("failed extracting information from custom configuration file")
-
 func (p *Provider) OpenVPNConfig(connection models.Connection,
 	settings settings.OpenVPN, ipv6Supported bool,
 ) (lines []string) {