gnucash future: Use ubuntu-26.04 for the doxygen workflow

[John Ralls]

Updated	 via  https://github.com/Gnucash/gnucash/commit/b76358c0 (commit)
	from  https://github.com/Gnucash/gnucash/commit/7a799fb4 (commit)



commit b76358c050c13fbd5fc26b76e7c936292dffd51f
Author: John Ralls <jralls at ceridwen.us>
Date:   Sat Aug 22 10:46:07 2026 -0700

    Use ubuntu-26.04 for the doxygen workflow
    
    Ubuntu-latest doesn't have cmake-4.0.



Summary of changes:
 .github/workflows/coverage.yml | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)