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/configuration/settings/provider.go

@@ -55,7 +55,7 @@ func (p *Provider) validate(vpnType string, filterChoicesGetter FilterChoicesGet
 		}
 	}
 	if err = validate.IsOneOf(p.Name, validNames...); err != nil {
-		return fmt.Errorf("%w for %s: %w", ErrVPNProviderNameNotValid, vpnType, err)
+		return fmt.Errorf("VPN provider name is not valid for %s: %w", vpnType, err)
 	}
 
 	err = p.ServerSelection.validate(p.Name, filterChoicesGetter, warner)