[WIP] Fix ban by software version, some other under-hood changes

- Use updated version of upstream go-xmpp + fixes on client-side
- Still waiting for https://github.com/xmppo/go-xmpp/pull/215
- Introduce golangci-lint v2 + fixes

vendor/github.com/xmppo/go-xmpp/xmpp_error.go

@@ -0,0 +1,57 @@
+package xmpp
+
+import (
+	"fmt"
+)
+
+// ErrorServiceUnavailable implements error response about a feature that is not available. Currently implemented for
+// xep-0030.
+// QueryXmlns is about incoming xmlns attribute in query tag.
+// Node is about incoming node attribute in query tag (looks like it used only in disco#commands).
+//
+// If queried feature is not here on purpose, standards suggest to answer with this stanza.
+func (c *Client) ErrorServiceUnavailable(v IQ, queryXmlns, node string) (string, error) {
+	query := fmt.Sprintf("<query xmlns='%s' ", queryXmlns)
+
+	if node != "" {
+		query += fmt.Sprintf("node='%s' />", node)
+	} else {
+		query += "/>"
+	}
+
+	query += "<error type='cancel'>"
+	query += "<service-unavailable xmlns='urn:ietf:params:xml:ns:xmpp-stanzas' />"
+	query += "</error>"
+
+	return c.RawInformation(
+		v.To,
+		v.From,
+		v.ID,
+		IQTypeError,
+		query,
+	)
+}
+
+// ErrorNotImplemented implements error response about a feature that is not (yet?) implemented.
+// Xmlns is about not implemented feature.
+//
+// If queried feature is not here because of it under development or for similar reasons, standards suggest to answer with
+// this stanza.
+func (c *Client) ErrorNotImplemented(v IQ, xmlns, feature string) (string, error) {
+	query := "<error type='cancel'>"
+	query += "<feature-not-implemented xmlns='urn:ietf:params:xml:ns:xmpp-stanzas' />"
+	query += fmt.Sprintf(
+		"<unsupported xmlns='%s' feature='%s' />",
+		xmlns,
+		feature,
+	)
+	query += "</error>"
+
+	return c.RawInformation(
+		v.To,
+		v.From,
+		v.ID,
+		IQTypeError,
+		query,
+	)
+}