Update CHANGES.txt and spark-ec2 and R package versions for 1.6.1

This patch updates a few more 1.6.0 version numbers for the 1.6.1 release candidate.

Verified this by running

```
git grep "1\.6\.0" | grep -v since | grep -v deprecated | grep -v Since | grep -v versionadded | grep 1.6.0
```

and inspecting the output.

Author: Josh Rosen <joshrosen@databricks.com>

Closes #11407 from JoshRosen/version-number-updates.