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/vpn.go

@@ -37,7 +37,7 @@ func (v *VPN) Validate(filterChoicesGetter FilterChoicesGetter, ipv6Supported bo
 	// Validate Type
 	validVPNTypes := []string{vpn.AmneziaWg, vpn.OpenVPN, vpn.Wireguard}
 	if err = validate.IsOneOf(v.Type, validVPNTypes...); err != nil {
-		return fmt.Errorf("%w: %w", ErrVPNTypeNotValid, err)
+		return fmt.Errorf("VPN type is not valid: %w", err)
 	}
 
 	err = v.Provider.validate(v.Type, filterChoicesGetter, warner)