Qt 5.12.8 for Focal

[Dmitry Shachnev]

Dear Release team,

Until a couple of weeks ago, we planned to update Qt to 5.14.2 for Focal.
But then 5.14.2 was delayed, and some doubts were raised about its stability,
so we changed our plan and decided to ship 5.12.8 instead.

5.12.8 is the latest release in 5.12 LTS series. Since 5.12.5 (the version
currently in Focal), the 5.12 branch has entered 'strict' phase, so it
receives only the selected important bug and security fixes [1]. In
particular, a lot of CVEs were fixed in Qt WebEngine.

5.12.8 was released two days ago, so I have just spent two days preparing it
in a PPA [2].

As with every Qt release, we need to rebuild all reverse dependencies that
use the private API (otherwise there would be a runtime error about versions
mismatch).

We will prepare all needed rebuilds in the PPA mentioned above, and we are
going to publish it in a few days. Hopefully that happens around Monday so
we still have a couple of days to let it migrate before the Final Freeze.

Please let me know about any concerns.

[1]: https://www.qt.io/blog/qt-5.12.5-released
[2]: https://launchpad.net/~ci-train-ppa-service/+archive/ubuntu/4005/+packages

--
Dmitry Shachnev