fix canonical tags to not point to now-deprecated docs

[wmorgan]

When we deprecate specific docs in a version, the canonical tags that we set in the older versions of those docs are pointing to the non-existing doc for the latest version.

E.g. there is no /2.12/tasks/securing-your-cluster/ doc (in this case, we renamed it for this version, but the same is true in earlier versions), but the 2.11, 2.10, etc versions still point to it in their meta rel=canoncial tags.

We should not create a canonical tag if the corresponding doc doesn't exist. (And for renames, we could potentially try and follow it but I think it would be hard to do, and I'm not sure it's really worth it.) The point of these canonical tags is to help Google index the latest version of everything by default, and if the latest version does not include this doc then we don't have to care too much about how it is indexed.