doc: update robots.txt to exclude more old docs

[dbkinder]

The current robots.txt excludes old docs (from 1.4.0 through 1.8.0) from search engines but is letting really old docs from 1.2.0 and 1.3.0 through. Add these two folder to the Disallow list too (or just delete some of these really old doc sets.

[rljordan-zz]

@dbkinder If you do not have access to do this, please let me know.

[dbkinder]

@rljordan I don't have access to do this. That's why I originally assigned it to Anas, since he does have access.

[nashif]

done