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/extract/data.go

@@ -1,17 +1,11 @@
 package extract
 
 import (
-	"errors"
 	"fmt"
 
 	"github.com/qdm12/gluetun/internal/models"
 )
 
-var (
-	ErrRead              = errors.New("cannot read file")
-	ErrExtractConnection = errors.New("cannot extract connection from file")
-)
-
 // Data extracts the lines and connection from the OpenVPN configuration file.
 func (e *Extractor) Data(filepath string) (lines []string,
 	connection models.Connection, err error,