doc: limit search engines scan on older content
Update robots.txt to exclude older documentation versions: 0.? 1.? and 2.0 through 2.4
User-agent: *
Allow: /
-Disallow: /0.1/
-Disallow: /0.2/
-Disallow: /0.3/
-Disallow: /0.4/
-Disallow: /0.5/
-Disallow: /0.6/
-Disallow: /0.7/
-Disallow: /0.8/
-Disallow: /1.0/
-Disallow: /1.1/
-Disallow: /1.2/
-Disallow: /1.3/
-Disallow: /1.4/
-Disallow: /1.5/
+Disallow: /0.?/
+Disallow: /1.?/
+Disallow: /2.0/
+Disallow: /2.1/
+Disallow: /2.2/
+Disallow: /2.3/
+Disallow: /2.4/
