doc: rephrase supported kernel versions

Change the wording in the readme so that we don't have to keep
updating it all the time. This also helps to set expectations with
users.

Updates #1434

Signed-off-by: Lorenz Bauer <lmb@isovalent.com>

README.md

@@ -58,8 +58,8 @@ This library includes the following packages:
 
 * A version of Go that is [supported by
   upstream](https://golang.org/doc/devel/release.html#policy)
-* Linux >= 4.9. CI is run against kernel.org LTS releases. 4.4 should work but is
-  not tested against.
+* CI is run against kernel.org LTS releases. >= 4.4 should work but EOL'ed versions
+  are not supported.
 
 ## License