Addressing the Python 2, AsciiDoc classic, and AsciiDoc 3 tangle

[James Browning]

I suggest that Asciidoc3 should be dropped as it is not
well-supported anywhere else and incurs additional maintenance
costs.

To address the support of AsciiDoc classic, we can use a Debian
Bullseye image until around July/August 2024, as it has AsciiDoc
classic. The current Debian Bookworm, which will replace
Bullseye, does not support AsciiDoc classic.

The image maintainer last updated the Bookworm and Bullseye
images in mid-December, while the Centos 6/7 and Python 2 image
maintainers last updated them a few years ago.

Fred is the only reliable support for Python 2.6, and we should
look for a maintained image that has Python 2.6, in case he is
unavailable.

We should consider dropping support for AsciiDoc3 and move the
Python 2.7 and AsciiDoc classic checks to Debian Bullseye. Also,
it has been proposed that we should drop support for Asciidoc
classic and Python 2 in August 2024 and 2025, respectively.