ci: remove Ubuntu 16.04

[github-actions[bot]]

Mega-Linter status: ✅ SUCCESS

Descriptor	Linter	Files	Fixed	Errors	Elapsed time
✅ CREDENTIALS	secretlint	yes		no	1.02s
✅ EDITORCONFIG	editorconfig-checker	1		0	0.81s
✅ GIT	git_diff	yes		no	0.01s
✅ SPELL	misspell	1		0	0.06s
✅ YAML	prettier	1		0	1.89s
✅ YAML	v8r	1		0	1.76s
✅ YAML	yamllint	1		0	0.22s

See errors details in artifact Mega-Linter reports on GitHub Action page _Set VALIDATE_ALL_CODEBASE: true in mega-linter.yml to validate all sources, not only the diff_