enhancements/update/targeted-update-edge-blocking: Propose a new enha…

…ncement

This enhancement proposes a mechanism for blocking edges for the
subset of clusters considered vulnerable to known issues with a
particular update or target release.

On the Markdown side, things are fairly standard, but I am using
'Text' for plain-text highlighting [1,2], which is new to me.

[1]: https://github.com/github/linguist/blob/16c70aef8cd62ca071231a380c69050f5e83c900/lib/linguist/languages.yml#L5868
[2]: https://docs.github.com/en/github/writing-on-github/working-with-advanced-formatting/creating-and-highlighting-code-blocks#syntax-highlighting