gnucash-docs stable: Remove Ubuntu-18.04 jobs from CI.

[John Ralls]

Updated	 via  https://github.com/Gnucash/gnucash-docs/commit/c4e1f0b4 (commit)
	from  https://github.com/Gnucash/gnucash-docs/commit/969de047 (commit)



commit c4e1f0b410796a0276e32eb5ac69ff34d79a457e
Author: John Ralls <jralls at ceridwen.us>
Date:   Sun Apr 16 11:29:51 2023 -0700

    Remove Ubuntu-18.04 jobs from CI.
    
    Github has removed Ubuntu-18.04 support and it's anyway redundant.



Summary of changes:
 .github/workflows/dist-check.yml   | 33 ---------------------------
 .github/workflows/html-ubuntu.yml  | 29 ------------------------
 .github/workflows/other-ubuntu.yml | 46 --------------------------------------
 3 files changed, 108 deletions(-)