Backport 6.9 to trixie

Update: I rebuilt this patch on top 6.9.1 in experimental. This was more work than I expected (lots of file changes in -dev files, icudtl.dat that appeared out of nowhere, an SBOM that seems to be using some new cmake functionality I could not find). This patchset works in my limited testing, but I'm only more confused now---I don't even see how 6.9.1+dfsg-3 builds, and am out of steam, hence the WIP.

Original, possibly out of date/wrong statements below:

I'm running this locally (albeit with patches generated by my mrs/security-20241004 script).

The upgrade to 6.9 went relatively smooth, and interacts (as far as I can tell) perfectly well with an otherwise bog-standard qt 6.8 setup.

There were some symbols removed from libqtwebenginecore, but I'm not sure how that is supposed to be handled.

In terms of security, I think it would be nice to keep a version that is closer to upstream's development branch, since that seems to receive security updates backported from Chromium more quickly.