Disable AI bots for older Qt documentation

doc.qt.io hosts also a lot of older documentation (see also http://doc.qt.io/sitemap.xml.gz).

For some of them, we probably have to keep them around if they're still supported. But we might block AI crawlers like Google Bart to index these, as there is more up-to-date information about these modules.

One way to do this for Google Bart is to block the new 'Google-Extended' user agent:

User-agent: Google-Extended
Disallow: /Boot2Qt-5.15/
Disallow: /Boot2Qt-6.2/
Disallow: /Boot2Qt-6.4/
Disallow: /GammaRay/
Disallow: /Neptune3UI-5.12/
/// etc