On a few of our buildbot instances, we get warnings about the usage of
deprecated functions. We should correct these, especially if we're
delegating to system versions of the libraries if they're available.
However, in order to do that, we need to update our library variant from
2.1.1 so that the non-deprecated alternatives are actually available.
The latest version has tons of security fixes (which is expected for a
library such as mbedtls).
Updating also allows getting rid of a few deprecation warnings.
