Dependency on vulnerable version of xmldom

`npm audit` reports:
```
xmldom  *
Severity: moderate
Misinterpretation of malicious XML input - https://npmjs.com/advisories/1769
No fix available
node_modules/xmldom
  node-saml  *
  Depends on vulnerable versions of xmldom
  node_modules/node-saml
```

The `xmldom` package has been succeeded by `@xmldom/xmldom`.